[Android.开发书籍] Learn Java for Android Development


Learn Java for Android Development Get the Java skills and know-how that you’ll need to learn and write successful Android apps Jeff “JavaJeff” FriesenThis book was purchased by harshamv@variable3.com i Learn Java for Android Development ■ ■ ■ Jeff “JavaJeff” Friesen ii Learn Java for Android Development Copyright © 2010 by Jeff “JavaJeff” Friesen All rights reserved. No part of this work may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval system, without the prior written permission of the copyright owner and the publisher. ISBN-13 (pbk): 978-1-4302-3156-1 ISBN-13 (electronic): 978-1-4302-3157-8 Printed and bound in the United States of America 9 8 7 6 5 4 3 2 1 Trademarked names, logos, and images may appear in this book. 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The source code for this book is available to readers at www.apress.com/book/view/1430231564. iii To my best friend iv Contents at a Glance ■Contents .......................................................................................................... v ■About the Author ............................................................................................. x ■About the Technical Reviewer........................................................................ xi ■Acknowledgments......................................................................................... xii ■Introduction.................................................................................................. xiii ■Chapter 1: Getting Started with Java .............................................................. 1 ■Chapter 2: Learning Language Fundamentals............................................... 43 ■Chapter 3: Learning Object-Oriented Language Features.............................. 97 ■Chapter 4: Mastering Advanced Language Features Part 1 ........................ 139 ■Chapter 5: Mastering Advanced Language Features Part 2 ........................ 181 ■Chapter 6: Exploring the Basic APIs Part 1 ................................................. 227 ■Chapter 7: Exploring the Basic APIs Part 2 ................................................. 269 ■Chapter 8: Discovering the Collections Framework .................................... 315 ■Chapter 9: Discovering Additional Utility APIs ............................................ 381 ■Chapter 10: Performing I/O ......................................................................... 449 ■Appendix: Solutions to Exercises ................................................................ 533 ■Index............................................................................................................ 595 v Contents ■Contents at a Glance....................................................................................... iv ■About the Author ............................................................................................. x ■About the Technical Reviewer........................................................................ xi ■Acknowledgments......................................................................................... xii ■Introduction.................................................................................................. xiii ■Chapter 1: Getting Started with Java .............................................................. 1 What Is Java?..........................................................................................................................................................1 Java Is a Language ............................................................................................................................................2 Java Is a Platform ..............................................................................................................................................3 Java SE, Java EE, Java ME, and Android...........................................................................................................5 Installing and Exploring the JDK.............................................................................................................................6 Installing and Exploring Two Popular IDEs............................................................................................................12 NetBeans IDE ...................................................................................................................................................13 Eclipse IDE .......................................................................................................................................................17 Four of a Kind........................................................................................................................................................20 Understanding Four of a Kind ..........................................................................................................................21 Modeling Four of a Kind in Pseudocode ..........................................................................................................21 Converting Pseudocode to Java Code..............................................................................................................23 Compiling, Running, and Distributing FourOfAKind .........................................................................................37 Summary ..............................................................................................................................................................41 ■Chapter 2: Learning Language Fundamentals............................................... 43 Classes..................................................................................................................................................................43 Declaring Classes ............................................................................................................................................44 Introducing Fields ............................................................................................................................................45 Introducing Methods........................................................................................................................................58 Introducing Constructors .................................................................................................................................75 Introducing Other Initializers............................................................................................................................76 Interface Versus Implementation.....................................................................................................................82 ■ CONTENTS vi Objects..................................................................................................................................................................85 Creating Objects and Arrays ............................................................................................................................85 Accessing Fields ..............................................................................................................................................87 Calling Methods ...............................................................................................................................................89 Garbage Collection...........................................................................................................................................92 Summary ..............................................................................................................................................................94 ■Chapter 3: Learning Object-Oriented Language Features.............................. 97 Inheritance............................................................................................................................................................97 Extending Classes............................................................................................................................................98 The Ultimate Superclass................................................................................................................................103 Composition...................................................................................................................................................112 The Trouble with Implementation Inheritance...............................................................................................112 Polymorphism.....................................................................................................................................................116 Upcasting and Late Binding...........................................................................................................................117 Abstract Classes and Abstract Methods ........................................................................................................120 Downcasting and Runtime Type Identification ..............................................................................................121 Covariant Return Types..................................................................................................................................123 Interfaces............................................................................................................................................................125 Declaring Interfaces.......................................................................................................................................125 Implementing Interfaces................................................................................................................................127 Extending Interfaces ......................................................................................................................................130 Why Use Interfaces? ......................................................................................................................................131 Summary ............................................................................................................................................................137 ■Chapter 4: Mastering Advanced Language Features Part 1 ........................ 139 Nested Types ......................................................................................................................................................139 Static Member Classes ..................................................................................................................................139 Nonstatic Member Classes ............................................................................................................................142 Anonymous Classes.......................................................................................................................................146 Local Classes .................................................................................................................................................148 Interfaces Within Classes ..............................................................................................................................150 Packages ............................................................................................................................................................151 What Are Packages?......................................................................................................................................151 The Package Statement.................................................................................................................................152 The Import Statement ....................................................................................................................................153 Searching for Packages and Types................................................................................................................154 Playing with Packages...................................................................................................................................155 Packages and JAR Files.................................................................................................................................159 Static Imports .....................................................................................................................................................160 Exceptions ..........................................................................................................................................................161 What Are Exceptions?....................................................................................................................................162 Representing Exceptions in Source Code ......................................................................................................162 Throwing Exceptions......................................................................................................................................166 Handling Exceptions ......................................................................................................................................168 Performing Cleanup .......................................................................................................................................172 Summary ............................................................................................................................................................178 ■Chapter 5: Mastering Advanced Language Features Part 2 ........................ 181 Assertions ...........................................................................................................................................................181 ■ CONTENTS vii Declaring Assertions......................................................................................................................................182 Using Assertions ............................................................................................................................................183 Avoiding Assertions .......................................................................................................................................188 Enabling and Disabling Assertions.................................................................................................................189 Annotations.........................................................................................................................................................190 Discovering Annotations ................................................................................................................................190 Declaring Annotation Types and Annotating Source Code.............................................................................193 Processing Annotations .................................................................................................................................198 Generics..............................................................................................................................................................200 Collections and the Need for Type Safety......................................................................................................200 Generic Types ................................................................................................................................................202 Generic Methods............................................................................................................................................212 Enums.................................................................................................................................................................214 The Trouble with Traditional Enumerated Types ...........................................................................................214 The Enum Alternative.....................................................................................................................................215 The Enum Class .............................................................................................................................................220 Summary ............................................................................................................................................................225 ■Chapter 6: Exploring the Basic APIs Part 1 ................................................. 227 Math APIs............................................................................................................................................................227 Math and StrictMath ......................................................................................................................................227 BigDecimal.....................................................................................................................................................234 BigInteger ......................................................................................................................................................239 Package Information...........................................................................................................................................243 Primitive Wrapper Classes..................................................................................................................................247 Boolean..........................................................................................................................................................248 Character .......................................................................................................................................................250 Float and Double............................................................................................................................................251 Integer, Long, Short, and Byte .......................................................................................................................255 Number ..........................................................................................................................................................257 References API....................................................................................................................................................257 Basic Terminology .........................................................................................................................................257 Reference and ReferenceQueue ....................................................................................................................259 SoftReference ................................................................................................................................................260 WeakReference..............................................................................................................................................263 PhantomReference ........................................................................................................................................263 Summary ............................................................................................................................................................268 ■Chapter 7: Exploring the Basic APIs Part 2 ................................................. 269 Reflection API......................................................................................................................................................269 String Management ............................................................................................................................................277 String .............................................................................................................................................................278 StringBuffer ...................................................................................................................................................281 System................................................................................................................................................................284 Threading API......................................................................................................................................................287 Runnable and Thread.....................................................................................................................................288 Thread Synchronization .................................................................................................................................296 Summary ............................................................................................................................................................313 ■Chapter 8: Discovering the Collections Framework .................................... 315■ CONTENTS viii Framework Overview..........................................................................................................................................315 Comparable Versus Comparator ....................................................................................................................316 Iterable and Collection ........................................................................................................................................318 Iterator and the Enhanced For Loop Statement.............................................................................................321 Autoboxing and Unboxing ..............................................................................................................................323 List ......................................................................................................................................................................325 ArrayList.........................................................................................................................................................329 LinkedList ......................................................................................................................................................330 Set.......................................................................................................................................................................332 TreeSet ..........................................................................................................................................................332 HashSet..........................................................................................................................................................333 EnumSet ........................................................................................................................................................337 SortedSet ............................................................................................................................................................339 Queue..................................................................................................................................................................346 PriorityQueue .................................................................................................................................................348 Map.....................................................................................................................................................................351 TreeMap.........................................................................................................................................................355 HashMap........................................................................................................................................................356 IdentityHashMap............................................................................................................................................362 WeakHashMap...............................................................................................................................................364 EnumMap.......................................................................................................................................................366 SortedMap ..........................................................................................................................................................367 Utilities................................................................................................................................................................369 Classic Collections Classes.................................................................................................................................372 Summary ............................................................................................................................................................379 ■Chapter 9: Discovering Additional Utility APIs ............................................ 381 Concurrency Utilities...........................................................................................................................................381 Executors .......................................................................................................................................................381 Synchronizers ................................................................................................................................................390 Concurrent Collections...................................................................................................................................392 Locks .............................................................................................................................................................394 Atomic Variables............................................................................................................................................397 Internationalization APIs .....................................................................................................................................397 Locales...........................................................................................................................................................398 Resource Bundles..........................................................................................................................................400 Break Iterators ...............................................................................................................................................409 Collators.........................................................................................................................................................413 Dates, Time Zones, and Calendars ................................................................................................................415 Formatters .....................................................................................................................................................421 Preferences API ..................................................................................................................................................428 Random Number Generation...............................................................................................................................432 Regular Expressions API .....................................................................................................................................434 Summary ............................................................................................................................................................447 ■Chapter 10: Performing I/O ......................................................................... 449 File ......................................................................................................................................................................449 RandomAccessFile..............................................................................................................................................462 Streams ..............................................................................................................................................................473 ■ CONTENTS ix Stream Classes Overview ..............................................................................................................................473 OutputStream and InputStream.....................................................................................................................475 ByteArrayOutputStream and ByteArrayInputStream......................................................................................477 FileOutputStream and FileInputStream..........................................................................................................479 PipedOutputStream and PipedInputStream ...................................................................................................481 FilterOutputStream and FilterInputStream.....................................................................................................485 BufferedOutputStream and BufferedInputStream..........................................................................................492 DataOutputStream and DataInputStream ......................................................................................................493 Object Serialization and Deserialization ........................................................................................................496 PrintStream....................................................................................................................................................508 Writers and Readers ...........................................................................................................................................511 Writer and Reader Classes Overview.............................................................................................................512 Writer and Reader..........................................................................................................................................513 OutputStreamWriter and InputStreamReader................................................................................................514 FileWriter and FileReader ..............................................................................................................................518 Summary ............................................................................................................................................................530 The Road Goes Ever On..................................................................................................................................530 ■Appendix: Solutions to Exercises ................................................................ 533 Chapter 1: Getting Started with Java..................................................................................................................533 Chapter 2: Learning Language Fundamentals ....................................................................................................539 Chapter 3: Learning Object-Oriented Language Features...................................................................................542 Chapter 4: Mastering Advanced Language Features Part 1................................................................................549 Chapter 5: Mastering Advanced Language Features Part 2................................................................................555 Chapter 6: Exploring the Basic APIs Part 1 .........................................................................................................560 Chapter 7: Exploring the Basic APIs Part 2 .........................................................................................................563 Chapter 8: Discovering the Collections Framework............................................................................................569 Chapter 9: Discovering Additional Utility APIs.....................................................................................................575 Chapter 10: Performing I/O .................................................................................................................................581 ■Index............................................................................................................ 595 ■ CONTENTS x About the Author Jeff “JavaJeff” Friesen has been actively involved with Java since the late 1990s. Jeff has worked with Java in various companies, including a healthcare-oriented consulting firm, where he created his own Java/C++ software for working with smart cards. Jeff has written about Java in numerous articles for JavaWorld (www.javaworld.com), informIT (www.informit.com), and java.net (http://java.net), and has authored several books on Java, including Beginning Java SE 6 Platform: From Novice to Professional (Apress, 2007; ISBN: 159059830X), which focuses exclusively on Java version 6’s new and improved features. Jeff has also taught Java in university and college continuing education classes. He has a Bachelor of Science degree in mathematics and computer science from Brandon University in Brandon, Manitoba, Canada, and currently freelances in Java and other software technologies. This book was purchased by harshamv@variable3.com ■ CONTENTS xi About the Technical Reviewer Paul Connolly is the Director of Engineering for Atypon Systems' RightSuite product line. RightSuite is an enterprise access-control and commerce solution used by many of the world's largest publishing and media companies. Paul enjoys designing and implementing high- performance, enterprise-class software systems. He is also an active contributor in the open-source community. Prior to joining Atypon Systems, Paul worked as a senior software engineer at Standard & Poor's where he architected and developed key communications systems. Paul is a Sun Certified Java Programmer, Sun Certified Business Component Developer, and a Sun Certified Web Component Developer. Paul lives in New York City with his wife, Marina. ■ CONTENTS xii Acknowledgments I thank Steve Anglin for contacting me to write this book, Debra Kelly for guiding me through the various aspects of this project, Tom Welsh for helping me with the development of my chapters, Paul Connolly for his diligence in catching various flaws that would otherwise have made it into this book, and Bill McManus and the production team for making the book’s content look good. It has been many years since I started writing about Java, and I also thank the following editors who have helped me share my knowledge with others: Chris Adamson, Bridget Collins, Richard Dal Porto, Sean Dixon, Victoria Elzey, Kevin Farnham, Todd Green, Jennifer Orr, Athen O’Shea, Esther Schindler, Daniel Steinberg, Jill Steinberg, Dustin Sullivan, and Atlanta Wilson. xiii Introduction Smartphones and other touch-based mobile devices are all the rage these days. Their popularity is largely due to their ability to run apps. Although the iPhone and iPad with their growing collection of Objective-C-based apps are the leaders of the pack, Android-based smartphones with their growing collection of Java-based apps are proving to be a strong competitor. Not only are many iPhone/iPad developers making money by selling their apps, many Android developers are also making money by selling similar apps. According to tech websites such as The Register (www.theregister.co.uk/), some Android developers are making lots of money (www.theregister.co.uk/2010/03/02/android_app_profit/). In today’s tough economic climate, perhaps you would like to try your hand at becoming an Android developer and make some money. If you have good ideas, perseverance, and some artistic talent (or perhaps know some talented individuals), you are already part of the way toward achieving this goal. Tip: A good reason to consider Android app development over iPhone/iPad app development is the lower startup costs that you will incur with Android. For example, you do not need to purchase a Mac on which to develop Android apps (a Mac is required for developing iPhone/iPad apps); your existing Windows, Linux, or Unix machine will do nicely. Most importantly, you will need to possess a solid understanding of the Java language and foundational application programming interfaces (APIs) before jumping into Android. After all, Android apps are written in Java and interact with many of the standard Java APIs (such as threading and input/output APIs). I wrote Learn Java for Android Development to give you a solid Java foundation that you can later extend with knowledge of Android architecture, API, and tool specifics. This book will give you a strong grasp of the Java language and many important APIs that are fundamental to Android apps and other Java applications. It will also introduce you to key development tools. Learn Java for Android Development is organized into ten chapters and one appendix. Each chapter focuses on a collection of related topics and presents a set of exercises that you should complete to get the most benefit from the chapter’s content. The appendix provides the solutions to each chapter’s exercises. ■ INTRODUCTION xiv Note: You can download this book’s source code by pointing your web browser to www.apress.com/book/view/1430231564 and clicking the Source Code link under Book Resources. Although most of this code is compilable with Java version 6, you will need Java version 7 to compile one of the applications. Chapter 1 introduces you to Java by first focusing on Java’s dual nature (language and platform). It then briefly introduces you to Sun’s/Oracle’s Java SE, Java EE, and Java ME editions of the Java development software, as well as Google’s Android edition. You next learn how to download and install the Java SE Development Kit (JDK), and learn some Java basics by developing and playing with a pair of simple Java applications. After receiving a brief introduction to the NetBeans and Eclipse IDEs, you learn about application development in the context of Four of a Kind, a console-based card game. Chapter 2 starts you on an in-depth journey of the Java language by focusing on language fundamentals (such as types, expressions, variables, and statements) in the contexts of classes and objects. Because applications are largely based on classes, it is important to learn how to architect classes correctly, and this chapter shows you how to do so. Chapter 3 adds to Chapter 2’s pool of object-based knowledge by introducing you to those language features that take you from object-based applications to object-oriented applications. Specifically, you learn about features related to inheritance, polymorphism, and interfaces. While exploring inheritance, you learn about Java’s ultimate superclass. Also, while exploring interfaces, you discover the real reason for their inclusion in the Java language; interfaces are not merely a workaround for Java’s lack of support for multiple implementation inheritance, but serve a higher purpose. Chapter 4 introduces you to four categories of advanced language features: nested types, packages, static imports, and exceptions. While discussing nested types, I briefly introduce the concept of a closure and state that closures will appear in Java version 7 (which many expect to arrive later this year). Note: I wrote this book several months before Java version 7’s expected arrival in the fall of 2010. Although I have tried to present an accurate portrayal of version 7–specific language features, it is possible that feature syntax may differ somewhat from what is presented in this book. Also, I only discuss closures briefly because this feature was still in a state of flux while writing this book. For more information about closures and other functional programming concepts (such as lambdas) being considered for Java version 7, I recommend that you check out articles such as “Functional Programming Concepts in JDK 7” by Alex Collins (http://java.dzone.com/articles/lambdas-closures-jdk-7). Chapter 5 continues to explore advanced language features by focusing on assertions, annotations, generics, and enums. Although the topic of generics has brought confusion to many developers, I believe that my discussion of this topic will clear up much of the murkiness. Among other items, you learn how to interpret type declarations such as Enum>. Chapter 6 begins a trend that focuses more on APIs than language features. This chapter first introduces you to many of Java’s math-oriented types (such as Math, StrictMath, BigDecimal, and BigInteger), and also introduces you to Java’s strictfp reserved word. It then looks at the Package class, primitive wrapper classes, and the References API. ■ INTRODUCTION xv Chapter 7 continues to explore Java’s basic APIs by focusing on reflection, string management, the System class, and threading. Chapter 8 focuses exclusively on Java’s collections framework, which provides you with a solution for organizing objects in lists, sets, queues, and maps. Chapter 9 continues to explore Java’s utility APIs by introducing you to the concurrency utilities, internationalization, preferences, random number generation, and regular expressions. Chapter 10 is all about input/output (I/O). In this chapter, you explore Java’s classic I/O support in terms of its File class, RandomAccessFile class, various stream classes, and various writer/reader classes. My discussion of stream I/O includes coverage of Java’s object serialization and deserialization mechanisms. Note: This book largely discusses APIs that are common to Java SE and Android. However, it diverges from this practice in Chapter 9, where I use the Swing toolkit to provide a graphical user interface for one of this chapter’s internationalization examples. (Android does not support Swing.) After you complete this book, I recommend that you obtain a copy of Beginning Android 2 by Mark L Murphy (Apress, 2010; ISBN: 1430226293) and start learning how to develop Android apps. In that book, “you’ll learn how to develop applications for Android 2.x mobile devices, using simple examples that are ready to run with your copy of the JDK.” Note: Over the next few months, I will make available at my javajeff.mb.ca website six additional PDF- based chapters. These chapters will introduce you to more Java APIs (such as networking and database APIs) that I could not discuss in this book because the book has greatly exceeded its initial 400-page estimate (and the good folks at Apress have been gracious enough to let me do so, but there are limits). I present more information about these PDF files at the end of Chapter 10’s “Summary” section. Thanks for purchasing my book. I hope you find it a helpful preparation for, and I wish you lots of success in achieving, a satisfying and lucrative career as an Android app developer. Jeff “JavaJeff” Friesen, August 2010 ■ INTRODUCTION xvi 1 1 Chapter Getting Started with Java Android is Google’s software stack for mobile devices that includes an operating system and middleware. With help from Java, the OS runs specially designed Java applications known as Android apps. Because these apps are based on Java, it makes sense for you to learn about Java before you dive into the world of Android development. NOTE: This book illustrates Java concepts via non-Android Java applications. This chapter sets the stage for teaching you the essential Java concepts that you need to understand before you embark on your Android career. I first answer the “What is Java?” question. I next show you how to install the Java SE Development Kit, and introduce you to JDK tools for compiling and running Java applications. After showing you how to install and use the open source NetBeans and Eclipse IDEs so that you can develop these applications faster, I present an application for playing a card game that I call Four of a Kind. This application gives you a significant taste of the Java language, and is the centerpiece of my discussion on developing applications. What Is Java? Java is a language and a platform originated by Sun Microsystems. This section briefly describes this language and reveals what it means for Java to be a platform. To meet various needs, Sun organized Java into three main editions: Java SE, Java EE, and Java ME. This section also briefly explores each of these editions, along with Android. 1 CHAPTER 1: Getting Started with Java 2 NOTE: Java has an interesting history that dates back to December 1990. At that time, James Gosling, Patrick Naughton, and Mike Sheridan (all employees of Sun Microsystems) were given the task of figuring out the next major trend in computing. They concluded that one trend would involve the convergence of computing devices and intelligent consumer appliances. Thus was born the Green project. The fruits of Green were Star7, a handheld wireless device featuring a five-inch color LCD screen, a SPARC processor, a sophisticated graphics capability, and a version of Unix; and Oak, a language developed by James Gosling for writing applications to run on Star7, and which he named after an oak tree growing outside of his office window at Sun. To avoid a conflict with another language of the same name, Dr. Gosling changed this language’s name to Java. Sun Microsystems subsequently evolved the Java language and platform until Oracle acquired Sun in early 2010. Check out http://java.sun.com/ for the latest Java news from Oracle. Java Is a Language Java is a language in which developers express source code (program text). Java’s syntax (rules for combining symbols into language features) is partly patterned after the C and C++ languages to shorten the learning curve for C/C++ developers. The following list identifies a few similarities between Java and C/C++:  Java and C/C++ share the same single-line and multiline comment styles. Comments let you document source code.  Many of Java’s reserved words are identical to their C/C++ counterparts (for, if, switch, and while are examples) and C++ counterparts (catch, class, public, and try are examples).  Java also supports character, double precision floating-point, floating- point, integer, long integer, and short integer primitive types, and via the same char, double, float, int, long, and short reserved words.  Java also supports many of the same operators, including arithmetic (+, -, *, /, and %) and conditional (?:) operators.  Java also uses brace characters ({ and }) to delimit blocks of statements. The following list identifies a few differences between Java and C/C++:  Java supports an additional comment style known as Javadoc. (I will briefly introduce Javadoc later in this chapter.)  Java provides reserved words not found in C/C++ (extends, strictfp, synchronized, and transient are examples). CHAPTER 1: Getting Started with Java 3  Java supports the byte integer type, does not provided a signed version of the character type, and does not provide unsigned versions of integer, long integer, and short integer. Furthermore, all of Java’s primitive types have guaranteed implementation sizes, which is an important part of achieving portability (discussed later). The same cannot be said of equivalent primitive types in C and C++.  Java provides operators not found in C/C++. These operators include instanceof and >>> (unsigned right shift).  Java provides labeled break and continue statements that you will not find in C/C++. You will learn about single-line and multiline comments in Chapter 2. Also, you will learn about reserved words, primitive types, operators, blocks, and statements (including labeled break and continue) in that chapter. Java was designed to be a safer language than C/C++. It achieves safety in part by omitting certain C/C++ features. For example, Java does not support pointers (variables containing addresses) and does not let you overload operators. Java also achieves safety by modifying certain C/C++ features. For example, loops must be controlled by Boolean expressions instead of integer expressions where 0 is false and a nonzero value is true. (Chapter 2 discusses loops and expressions.) Suppose you must code a C/C++ while loop that repeats no more than ten times. Being tired, you specify while (x) x++; (assume that x is an integer-based variable initialized to 0—I discuss variables in Chapter 2) where x++ adds 1 to x’s value. This loop does not stop when x reaches 10; you have introduced a bug (a defect). This problem is less likely to occur in Java because it complains when it sees while (x). This complaint requires you to recheck your expression, and you will then most likely specify while (x != 10). Not only is safety improved (you cannot specify just x), meaning is also clarified: while (x != 10) is more meaningful than while (x). The aforementioned and other fundamental language features support classes, objects, inheritance, polymorphism, and interfaces. Java also provides advanced features related to nested types, packages, static imports, exceptions, assertions, annotations, generics, enums, and more. Subsequent chapters explore all of these language features. Java Is a Platform Java is a platform for executing programs. In contrast to platforms that consist of physical processors (such as an Intel processor) and operating systems (such as Linux), the Java platform consists of a virtual machine and associated execution environment. The virtual machine is a software-based processor that presents its own instruction set. The associated execution environment consists of libraries for running programs and interacting with the underlying operating system. CHAPTER 1: Getting Started with Java 4 The execution environment includes a huge library of prebuilt classfiles that perform common tasks, such as math operations (trigonometry, for example) and network communications. This library is commonly referred to as the standard class library. A special Java program known as the Java compiler translates source code into instructions (and associated data) that are executed by the virtual machine. These instructions are commonly referred to as bytecode. The compiler stores a program’s bytecode and data in files having the .class extension. These files are known as classfiles because they typically store the compiled equivalent of classes, a language feature discussed in Chapter 2. A Java program executes via a tool (such as java) that loads and starts the virtual machine, and passes the program’s main classfile to the machine. The virtual machine uses a classloader (a virtual machine or execution environment component) to load the classfile. After the classfile has been loaded, the virtual machine’s bytecode verifier component makes sure that the classfile’s bytecode is valid and does not compromise security. The verifier terminates the virtual machine when it finds a problem with the bytecode. Assuming that all is well with the classfile’s bytecode, the virtual machine’s interpreter interprets the bytecode one instruction at a time. Interpretation consists of identifying bytecode instructions and executing equivalent native instructions. NOTE: Native instructions (also known as native code) are the instructions understood by the underlying platform’s physical processor. When the interpreter learns that a sequence of bytecode instructions is executed repeatedly, it informs the virtual machine’s Just In Time (JIT) compiler to compile these instructions into native code. JIT compilation is performed only once for a given sequence of bytecode instructions. Because the native instructions execute instead of the associated bytecode instruction sequence, the program executes much faster. During execution, the interpreter might encounter a request to execute another classfile’s bytecode. When that happens, it asks the classloader to load the classfile and the bytecode verifier to verify the bytecode prior to executing that bytecode. The platform side of Java promotes portability by providing an abstraction over the underlying platform. As a result, the same bytecode runs unchanged on Windows- based, Linux-based, Mac OS X–based, and other platforms. This book was purchased by harshamv@variable3.comCHAPTER 1: Getting Started with Java 5 NOTE: Java was introduced with the “write once, run anywhere” slogan. Although Java goes to great lengths to enforce portability, it does not always succeed. Despite being mostly platform independent, certain parts of Java (such as the scheduling of threads, discussed in Chapter 7) vary from underlying platform to underlying platform. The platform side of Java also promotes security by providing a secure environment in which code executes. The goal is to prevent malicious code from corrupting the underlying platform (and possibly stealing sensitive information). NOTE: Because many developers are not satisfied with the Java language, but believe that the Java platform is important, they have devised additional languages (such as Groovy) that run on the Java platform. Furthermore, Java version 7 includes an enhanced virtual machine that simplifies adapting even more dynamic programming languages (languages that require less- rigid coding; you do not have to define a variable’s type before using the variable, for example) to this platform. Java SE, Java EE, Java ME, and Android Developers use different editions of the Java platform to create Java programs that run on desktop computers, web browsers, web servers, mobile information devices (such as cell phones), and embedded devices (such as television set-top boxes):  Java Platform, Standard Edition (Java SE): The Java platform for developing applications, which are stand-alone programs that run on desktops. Java SE is also used to develop applets, which are programs that run in the context of a web browser.  Java Platform, Enterprise Edition (Java EE): The Java platform for developing enterprise-oriented applications and servlets, which are server programs that conform to Java EE’s Servlet API. Java EE is built on top of Java SE.  Java Platform, Micro Edition (Java ME): The Java platform for developing MIDlets, which are programs that run on mobile information devices, and Xlets, which are programs that run on embedded devices. Developers also use a special Google-created edition of the Java platform (see http://developer.android.com/index.html) to create Android apps that run on Android-enabled devices. This edition is known as the Android platform. Google’s Android platform largely consists of Java core libraries (partly based on Java SE) and a virtual machine known as Dalvik. This collective software runs on top of a specially modified Linux kernel. CHAPTER 1: Getting Started with Java 6 NOTE: Check out Wikipedia’s “Android (operating system)” entry (http://en.wikipedia.org/wiki/Android_%28operating_system%29) to learn more about the Android OS, and Wikipedia’s “Dalvik (software)” entry (http://en.wikipedia.org/wiki/Dalvik_%28software%29) to learn more about the Dalvik virtual machine. In this book, I cover the Java language (supported by Java SE and Android) and Java SE APIs (also supported by Android). Furthermore, I present the source code (typically as code fragments) to Java SE–based applications. Installing and Exploring the JDK The Java Runtime Environment (JRE) implements the Java SE platform and makes it possible to run Java programs. The public JRE can be downloaded from the Java SE Downloads page (http://java.sun.com/javase/downloads/index.jsp). However, the public JRE does not make it possible to develop Java programs. For that task, you need to download and install the Java SE Development Kit (JDK), which contains development tools (including the Java compiler) and a private JRE. NOTE: JDK 1.0 was the first JDK to be released (in May 1995). Until JDK 6 arrived, JDK stood for Java Development Kit (SE was not part of the title). Over the years, numerous JDKs have been released, with JDK 7 set for release in fall or winter 2010. Each JDK’s version number identifies a version of Java. For example, JDK 1.0 identifies Java version 1.0, and JDK 5 identifies Java version 5.0. JDK 5 was the first JDK to also provide an internal version number: 1.5.0. The Java SE Downloads page also provides access to the current JDK, which is JDK 6 Update 20 at time of writing. Click the Download JDK link to download the current JDK’s installer program for your platform. NOTE: Some of this book’s code requires JDK 7, which is only available as a preview release (http://java.sun.com/javase/downloads/ea.jsp) at time of writing. The JDK installer installs the JDK in a home directory. (It can also install the public JRE in another directory.) On my Windows XP platform, the home directory is C:\Program Files\Java\jdk1.6.0_16—JDK 6 Update 16 was current when I began this book. CHAPTER 1: Getting Started with Java 7 TIP: After installing the JDK, you should add the bin subdirectory to your platform’s PATH environment variable. That way, you will be able to execute JDK tools from any directory in your filesystem. Finally, you might want to create a projects subdirectory of the JDK’s home directory to organize your Java projects, and create a separate subdirectory within projects for each of these projects. The home directory contains various files (such as README.html, which provides information about the JDK, and src.zip, which provides the standard class library source code) and subdirectories, including the following three important subdirectories:  bin: This subdirectory contains assorted JDK tools, including the Java compiler tool. You will discover some of these tools shortly.  jre: This subdirectory contains the JDK’s private copy of the JRE, which lets you run Java programs without having to download and install the public JRE.  lib: This subdirectory contains library files that are used by JDK tools. For example, tools.jar contains the Java compiler’s classfiles—the compiler was written in Java. You will use only a few of the bin subdirectory’s tools in this book, specifically javac (Java compiler), java (Java application launcher), javadoc (Java documentation generator), and jar (Java archive creator, updater, and extractor). NOTE: javac is not the Java compiler. It is a tool that loads and starts the virtual machine, identifies the compiler’s main classfile (located in tools.jar) to the virtual machine, and passes the name of the source file being compiled to the compiler’s main classfile. You execute JDK tools at the command line, passing command-line arguments to a tool. Learn about the command line and arguments via Wikipedia’s “Command-line interface” entry (http://en.wikipedia.org/wiki/Command-line_interface). Now that you have installed the JDK and know something about its tools, you are ready to explore a small DumpArgs application that outputs its command-line arguments to the standard output device. CHAPTER 1: Getting Started with Java 8 NOTE: The standard output device is part of a mechanism known as Standard I/O. This mechanism, which consists of Standard Input, Standard Output, and Standard Error, and which originated with the Unix operating system, makes it possible to read text from different sources (keyboard or file) and write text to different destinations (screen or file). Text is read from the standard input device, which defaults to the keyboard but can be redirected to a file. Text is output to the standard output device, which defaults to the screen but can be redirected to a file. Error message text is output to the standard error device, which defaults to the screen but can be redirected to a file that differs from the standard output file. Listing 1–1 presents the DumpArgs application source code. Listing 1–1. Dumping command-line arguments via main()’s args array to the standard output device public class DumpArgs { public static void main(String[] args) { System.out.println("Passed arguments:"); for (int i = 0; i < args.length; i++) System.out.println(args[i]); } } Listing 1–1’s DumpArgs application consists of a class named DumpArgs and a method within this class named main(), which is the application’s entry point and provides the code to execute. (You will learn about classes and methods in Chapter 2.) main() is called with an array of strings (character sequences) that identify the application’s command-line arguments. These strings are stored in String-based array variable args. (I discuss method calling, arrays, and variables in Chapter 2.) NOTE: Although the array variable is named args, there is nothing special about this name. You could name this variable anything you want. main() first executes System.out.println("Passed arguments:");, which calls System.out’s println() method with the "Passed arguments:" string. This method call outputs Passed arguments: to the standard output device and then terminates the current line so that subsequent output is sent to a new line immediately below the current line. (I discuss System.out in Chapter 7.) CHAPTER 1: Getting Started with Java 9 NOTE: System.out provides access to a family of println() methods and a family of print() methods for outputting different kinds of data (such as sequences of characters and integers). Unlike the println() methods, the print() methods do not terminate the current line; subsequent output continues on the current line. Each println() method terminates a line by outputting a line separator string, which is defined by system property line.separator, and which is not necessarily a single newline character (identified in source code via character literal '\n'). (I discuss system properties in Chapter 7, line.separator in Chapter 10, and character literals in Chapter 2.) For example, on Windows platforms, the line separator string is a carriage return character (whose integer code is 13) followed by a line feed character (whose integer code is 10). main() uses a for loop to repeatedly execute System.out.println(args[i]);. The loop executes args.length times, which happens to identify the number of strings that are stored in args. (I discuss for loops in Chapter 2.) The System.out.println(args[i]); method call reads the string stored in the ith entry of the args array—the first entry is located at index (location) 0; the last entry is stored at index args.length - 1. This method call then outputs this string to standard output. Assuming that you are familiar with your platform’s command-line interface and are at the command line, make DumpArgs your current directory and copy Listing 1–1 to a file named DumpArgs.java. Then compile this source file via the following command line: javac DumpArgs.java Assuming that that you have included the .java extension, which is required by javac, and that DumpArgs.java compiles, you should discover a file named DumpArgs.class in the current directory. Run this application via the following command line: java DumpArgs If all goes well, you should see the following line of output on the screen: Passed arguments: For more interesting output, you will need to pass command-line arguments to DumpArgs. For example, execute the following command line, which specifies Curly, Moe, and Larry as three arguments to pass to DumpArgs: java DumpArgs Curly Moe Larry This time, you should see the following expanded output on the screen: Passed arguments: Curly Moe Larry CHAPTER 1: Getting Started with Java 10 You can redirect the output destination to a file by specifying the greater than angle bracket (>) followed by a filename. For example, java DumpArgs Curly Moe Larry >out.txt stores the DumpArgs application’s output in a file named out.txt. NOTE: Instead of specifying System.out.println(), you could specify System.err.println() to output characters to the standard error device. (System.err provides the same families of println() and print() methods as System.out.) However, you should only switch from System.out to System.err when you need to output an error message so that the error messages are displayed on the screen, even when standard output is redirected to a file. Congratulations on successfully compiling your first application source file and running the application! Listing 1–2 presents the source code to a second application, which echoes text obtained from the standard input device to the standard output device. Listing 1–2. Echoing text read from standard input to standard output public class EchoText { public static void main(String[] args) throws java.io.IOException { System.out.println("Please enter some text and press Enter!"); int ch; while ((ch = System.in.read()) != 13) System.out.print((char) ch); System.out.println(); } } After outputting a message that prompts the user to enter some text, main() introduces int variable ch to store each character’s integer representation. (You will learn about int and integer in Chapter 2.) main() now enters a while loop (discussed in Chapter 2) to read and echo characters. The loop first calls System.in.read() to read a character and assign its integer value to ch. The loop ends when this value equals 13 (the integer value of the Enter key). NOTE: When standard input is not redirected to a file, System.in.read() returns 13 to indicate that the Enter key has been pressed. On platforms such as Windows, a subsequent call to System.in.read() returns integer 10, indicating a line feed character. Whether or not standard input has been redirected, System.in.read() returns -1 when there are no more characters to read. For any other value in ch, this value is converted to a character via (char), which is an example of Java’s cast operator (discussed in Chapter 2). The character is then output via System.out.println(). The final System.out.println(); call terminates the current line without outputting any content. CHAPTER 1: Getting Started with Java 11 NOTE: When standard input is redirected to a file and System.in.read() is unable to read text from the file (perhaps the file is stored on a removable storage device that has been removed prior to the read operation), System.in.read() fails by throwing an object that describes this problem. I acknowledge this possibility by appending throws java.io.IOException to the end of the main() method header. I discuss throws in Chapter 4 and java.io.IOException in Chapter 10. Compile Listing 1–2 via javac EchoText.java, and run the application via java EchoText. You will be prompted to enter some text. After you input this text and press Enter, the text will be sent to standard output. For example, consider the following output: Please enter some text and press Enter! Hello Java Hello Java You can redirect the input source to a file by specifying the less than angle bracket (<) followed by a filename. For example, java EchoText hCard.rank() 7.1.1. curPlayer = COMPUTER 8. deck.putBack(hCard) 9. deck.putBack(cCard) 10. if curPlayer == HUMAN 10.1. for i = 0 to 3 10.1.1. cCards[i] = deck.deal() 10.1.2. hCards[i] = deck.deal() else 10.2. for i = 0 to 3 10.2.1. hCards[i] = deck.deal() 10.2.2. cCards[i] = deck.deal() 11. if curPlayer == HUMAN 11.01. output(hCards) 11.02. choice = prompt("Identify card to throw away") 11.03. discardPile.setTopCard(hCards[choice]) 11.04. hCards[choice] = deck.deal() 11.05. if isFourOfAKind(hCards) 11.05.1. output("Human wins!") 11.05.2. putDown(hCards) 11.05.3. output("Computer's cards:") 11.05.4. putDown(cCards) 11.05.5. End game 11.06. curPlayer = COMPUTER CHAPTER 1: Getting Started with Java 23 else 11.07. choice = leastDesirableCard(cCards) 11.08. discardPile.setTopCard(cCards[choice]) 11.09. cCards[choice] = deck.deal() 11.10. if isFourOfAKind(cCards) 11.10.1. output("Computer wins!") 11.10.2. putDown(cCards) 11.10.3. End game 11.11. curPlayer = HUMAN 12. if deck.isEmpty() 12.1. if discardPile.topCard() != null 12.1.1. deck.putBack(discardPile.getTopCard()) 12.1.2. Repeat at step 12.1. 12.2. deck.shuffle() 13. Repeat at step 11. In addition to being longer than Listing 1–3, Listing 1–4 shows the refined pseudocode becoming more like Java. For example, Listing 1–4 reveals Java expressions (such as new Deck(), to create a Deck object), operators (such as ==, to compare two values for equality), and method calls (such as deck.isEmpty(), to call deck’s isEmpty() method to return a Boolean value indicating whether [true] or not [false] the deck identified by deck is empty of cards). Converting Pseudocode to Java Code Now that you have had a chance to absorb Listing 1–4’s Java-like pseudocode, you are ready to examine the process of converting that pseudocode to Java source code. This process consists of a couple of steps. The first step in converting Listing 1–4’s pseudocode to Java involves identifying important components of the game’s structure and implementing these components as classes. I will formally introduce classes in Chapter 2. Apart from the computer player (which is implemented via game logic), the important components are card, deck, and discard pile. I represent these components via Card, Deck, and DiscardPile classes. Listing 1–5 presents Card. NOTE: Do not be concerned if you find this section’s Java source code somewhat hard to follow. After you have read the next few chapters, you should find this code easier to understand. Listing 1–5. Merging suits and ranks into cards /** * Simulating a playing card. * * @author Jeff Friesen */ public enum Card { ACE_OF_CLUBS(Suit.CLUBS, Rank.ACE), TWO_OF_CLUBS(Suit.CLUBS, Rank.TWO), CHAPTER 1: Getting Started with Java 24 THREE_OF_CLUBS(Suit.CLUBS, Rank.THREE), FOUR_OF_CLUBS(Suit.CLUBS, Rank.FOUR), FIVE_OF_CLUBS(Suit.CLUBS, Rank.FIVE), SIX_OF_CLUBS(Suit.CLUBS, Rank.SIX), SEVEN_OF_CLUBS(Suit.CLUBS, Rank.SEVEN), EIGHT_OF_CLUBS(Suit.CLUBS, Rank.EIGHT), NINE_OF_CLUBS(Suit.CLUBS, Rank.NINE), TEN_OF_CLUBS(Suit.CLUBS, Rank.TEN), JACK_OF_CLUBS(Suit.CLUBS, Rank.JACK), QUEEN_OF_CLUBS(Suit.CLUBS, Rank.QUEEN), KING_OF_CLUBS(Suit.CLUBS, Rank.KING), ACE_OF_DIAMONDS(Suit.DIAMONDS, Rank.ACE), TWO_OF_DIAMONDS(Suit.DIAMONDS, Rank.TWO), THREE_OF_DIAMONDS(Suit.DIAMONDS, Rank.THREE), FOUR_OF_DIAMONDS(Suit.DIAMONDS, Rank.FOUR), FIVE_OF_DIAMONDS(Suit.DIAMONDS, Rank.FIVE), SIX_OF_DIAMONDS(Suit.DIAMONDS, Rank.SIX), SEVEN_OF_DIAMONDS(Suit.DIAMONDS, Rank.SEVEN), EIGHT_OF_DIAMONDS(Suit.DIAMONDS, Rank.EIGHT), NINE_OF_DIAMONDS(Suit.DIAMONDS, Rank.NINE), TEN_OF_DIAMONDS(Suit.DIAMONDS, Rank.TEN), JACK_OF_DIAMONDS(Suit.DIAMONDS, Rank.JACK), QUEEN_OF_DIAMONDS(Suit.DIAMONDS, Rank.QUEEN), KING_OF_DIAMONDS(Suit.DIAMONDS, Rank.KING), ACE_OF_HEARTS(Suit.HEARTS, Rank.ACE), TWO_OF_HEARTS(Suit.HEARTS, Rank.TWO), THREE_OF_HEARTS(Suit.HEARTS, Rank.THREE), FOUR_OF_HEARTS(Suit.HEARTS, Rank.FOUR), FIVE_OF_HEARTS(Suit.HEARTS, Rank.FIVE), SIX_OF_HEARTS(Suit.HEARTS, Rank.SIX), SEVEN_OF_HEARTS(Suit.HEARTS, Rank.SEVEN), EIGHT_OF_HEARTS(Suit.HEARTS, Rank.EIGHT), NINE_OF_HEARTS(Suit.HEARTS, Rank.NINE), TEN_OF_HEARTS(Suit.HEARTS, Rank.TEN), JACK_OF_HEARTS(Suit.HEARTS, Rank.JACK), QUEEN_OF_HEARTS(Suit.HEARTS, Rank.QUEEN), KING_OF_HEARTS(Suit.HEARTS, Rank.KING), ACE_OF_SPADES(Suit.SPADES, Rank.ACE), TWO_OF_SPADES(Suit.SPADES, Rank.TWO), THREE_OF_SPADES(Suit.SPADES, Rank.THREE), FOUR_OF_SPADES(Suit.SPADES, Rank.FOUR), FIVE_OF_SPADES(Suit.SPADES, Rank.FIVE), SIX_OF_SPADES(Suit.SPADES, Rank.SIX), SEVEN_OF_SPADES(Suit.SPADES, Rank.SEVEN), EIGHT_OF_SPADES(Suit.SPADES, Rank.EIGHT), NINE_OF_SPADES(Suit.SPADES, Rank.NINE), TEN_OF_SPADES(Suit.SPADES, Rank.TEN), JACK_OF_SPADES(Suit.SPADES, Rank.JACK), QUEEN_OF_SPADES(Suit.SPADES, Rank.QUEEN), KING_OF_SPADES(Suit.SPADES, Rank.KING); private Suit suit; /** * Return Card's suit. * * @return CLUBS, DIAMONDS, HEARTS, * or SPADES This book was purchased by harshamv@variable3.comCHAPTER 1: Getting Started with Java 25 */ public Suit suit() { return suit; } private Rank rank; /** * Return Card's rank. * * @return ACE, TWO, THREE, * FOUR, FIVE, SIX, * SEVEN, EIGHT, NINE, * TEN, JACK, QUEEN, * KING. */ public Rank rank() { return rank; } Card(Suit suit, Rank rank) { this.suit = suit; this.rank = rank; } /** * A card's suit is its membership. * * @author Jeff Friesen */ public enum Suit { CLUBS, DIAMONDS, HEARTS, SPADES } /** * A card's rank is its integer value. * * @author Jeff Friesen */ public enum Rank { ACE, TWO, THREE, FOUR, FIVE, SIX, SEVEN, EIGHT, NINE, TEN, JACK, QUEEN, KING } } Listing 1–5 begins with a Javadoc comment that is used to briefly describe the Card class and identify this class’s author. I will introduce you to Javadoc comments at the end of this section. NOTE: One feature of Javadoc comments is the ability to embed HTML tags. These tags specify different kinds of formatting for sections of text within these comments. For example, and specify that their enclosed text is to be formatted as a code listing. Later in this chapter, you will learn how to convert these comments into HTML documentation. Card is an example of an enum, which is a special kind of class that I discuss in Chapter 5. For now, think of Card as a place to create and store Card objects that identify all 52 cards that make up a standard deck. CHAPTER 1: Getting Started with Java 26 Card declares a nested Suit enum. (I discuss nesting in Chapter 4.) A card’s suit denotes its membership. The only legal Suit values are CLUBS, DIAMONDS, HEARTS, and SPADES. Card also declares a nested Rank enum. A card’s rank denotes its value: ACE, TWO, THREE, FOUR, FIVE, SIX, SEVEN, EIGHT, NINE, TEN, JACK, QUEEN, and KING are the only legal Rank values. A Card object is created when Suit and Rank objects are passed to its constructor. (I discuss constructors in Chapter 2.) For example, KING_OF_HEARTS(Suit.HEARTS, Rank.KING) combines Suit.HEARTS and Rank.KING into KING_OF_HEARTS. Card provides a rank() method for returning a Card’s Rank object. Similarly, Card provides a suit() method for returning a Card’s Suit object. For example, KING_OF_HEARTS.rank() returns Rank.KING, and KING_OF_HEARTS.suit() returns Suit.HEARTS. Listing 1–6 presents the Java source code to the Deck class, which implements a deck of 52 cards. Listing 1–6. Pick a card, any card import java.util.ArrayList; import java.util.Collections; import java.util.List; /** * Simulate a deck of cards. * * @author Jeff Friesen */ public class Deck { private Card[] cards = new Card[] { Card.ACE_OF_CLUBS, Card.TWO_OF_CLUBS, Card.THREE_OF_CLUBS, Card.FOUR_OF_CLUBS, Card.FIVE_OF_CLUBS, Card.SIX_OF_CLUBS, Card.SEVEN_OF_CLUBS, Card.EIGHT_OF_CLUBS, Card.NINE_OF_CLUBS, Card.TEN_OF_CLUBS, Card.JACK_OF_CLUBS, Card.QUEEN_OF_CLUBS, Card.KING_OF_CLUBS, Card.ACE_OF_DIAMONDS, Card.TWO_OF_DIAMONDS, Card.THREE_OF_DIAMONDS, Card.FOUR_OF_DIAMONDS, Card.FIVE_OF_DIAMONDS, Card.SIX_OF_DIAMONDS, Card.SEVEN_OF_DIAMONDS, Card.EIGHT_OF_DIAMONDS, Card.NINE_OF_DIAMONDS, CHAPTER 1: Getting Started with Java 27 Card.TEN_OF_DIAMONDS, Card.JACK_OF_DIAMONDS, Card.QUEEN_OF_DIAMONDS, Card.KING_OF_DIAMONDS, Card.ACE_OF_HEARTS, Card.TWO_OF_HEARTS, Card.THREE_OF_HEARTS, Card.FOUR_OF_HEARTS, Card.FIVE_OF_HEARTS, Card.SIX_OF_HEARTS, Card.SEVEN_OF_HEARTS, Card.EIGHT_OF_HEARTS, Card.NINE_OF_HEARTS, Card.TEN_OF_HEARTS, Card.JACK_OF_HEARTS, Card.QUEEN_OF_HEARTS, Card.KING_OF_HEARTS, Card.ACE_OF_SPADES, Card.TWO_OF_SPADES, Card.THREE_OF_SPADES, Card.FOUR_OF_SPADES, Card.FIVE_OF_SPADES, Card.SIX_OF_SPADES, Card.SEVEN_OF_SPADES, Card.EIGHT_OF_SPADES, Card.NINE_OF_SPADES, Card.TEN_OF_SPADES, Card.JACK_OF_SPADES, Card.QUEEN_OF_SPADES, Card.KING_OF_SPADES }; private List deck; /** * Create a Deck of 52 Card objects. Shuffle * these objects. */ public Deck() { deck = new ArrayList(); for (int i = 0; i < cards.length; i++) { deck.add(cards[i]); cards[i] = null; } Collections.shuffle(deck); } /** * Deal the Deck's top Card object. * * @return the Card object at the top of the * Deck */ public Card deal() { return deck.remove(0); } /** CHAPTER 1: Getting Started with Java 28 * Return an indicator of whether or not the Deck is empty. * * @return true if the Deck contains no Card * objects; otherwise, false */ public boolean isEmpty() { return deck.isEmpty(); } /** * Put back a Card at the bottom of the Deck. * * @param card Card object being put back */ public void putBack(Card card) { deck.add(card); } /** * Shuffle the Deck. */ public void shuffle() { Collections.shuffle(deck); } } Deck initializes a private cards array to all 52 Card objects. Because it is easier to implement Deck via a list that stores these objects, Deck’s constructor creates this list and adds each Card object to the list. (I discuss List and ArrayList in Chapter 8.) Deck also provides deal(), isEmpty(), putBack(), and shuffle() methods to deal a single Card from the Deck (the Card is physically removed from the Deck), determine whether or not the Deck is empty, put a Card back into the Deck, and shuffle the Deck’s Cards. Listing 1–7 presents the source code to the DiscardPile class, which implements a discard pile on which players can throw away a maximum of 52 cards. Listing 1–7. A garbage dump for cards /** * Simulate a pile of discarded cards. * * @author Jeff Friesen */ public class DiscardPile { private Card[] cards; private int top; /** * Create a DiscardPile that can accommodate a maximum of 52 * Cards. The DiscardPile is initially empty. */ public DiscardPile() { cards = new Card[52]; // Room for entire deck on discard pile (should CHAPTER 1: Getting Started with Java 29 // never happen). top = -1; } /** * Return the Card at the top of the DiscardPile. * * @return Card object at top of DiscardPile or * null if DiscardPile is empty */ public Card getTopCard() { if (top == -1) return null; Card card = cards[top]; cards[top--] = null; return card; } /** * Set the DiscardPile's top card to the specified * Card object. * * @param card Card object being thrown on top of the * DiscardPile */ public void setTopCard(Card card) { cards[++top] = card; } /** * Identify the top Card on the DiscardPile * without removing this Card. * * @return top Card, or null if DiscardPile is * empty */ public Card topCard() { return (top == -1) ? null : cards[top]; } } DiscardPile implements a discard pile on which to throw Card objects. It implements the discard pile via a stack metaphor: the last Card object thrown on the pile sits at the top of the pile and is the first Card object to be removed from the pile. This class stores its stack of Card objects in a private cards array. I found it convenient to specify 52 as this array’s storage limit because the maximum number of Cards is 52. (Game play will never result in all Cards being stored in the array.) Along with its constructor, DiscardPile provides getTopCard(), setTopCard(), and topCard() methods to remove and return the stack’s top Card, store a new Card object on the stack as its top Card, and return the top Card without removing it from the stack. The constructor demonstrates a single-line comment, which starts with the // character sequence. This comment documents that the cards array has room to store the entire Deck of Cards. I will formally introduce single-line comments in Chapter 2. CHAPTER 1: Getting Started with Java 30 The second step in converting Listing 1–4’s pseudocode to Java involves introducing a FourOfAKind class whose main() method contains the Java code equivalent of this pseudocode. Listing 1–8 presents FourOfAKind. Listing 1–8. FourOfAKind application source code /** * FourOfAKind implements a card game that is played between two * players: one human player and the computer. You play this game with a * standard 52-card deck and attempt to beat the computer by being the first * player to put down four cards that have the same rank (four aces, for * example), and win. * *

* The game begins by shuffling the deck and placing it face down. Each * player takes a card from the top of the deck. The player with the highest * ranked card (king is highest) deals four cards to each player starting * with the other player. The dealer then starts its turn. * *

* The player examines its cards to determine which cards are optimal for * achieving four of a kind. The player then throws away one card on a * discard pile and picks up another card from the top of the deck. If the * player has four of a kind, the player puts down these cards (face up) and * wins the game. * * @author Jeff Friesen * @version 1.0 */ public class FourOfAKind { /** * Human player */ final static int HUMAN = 0; /** * Computer player */ final static int COMPUTER = 1; /** * Application entry point. * * @param args array of command-line arguments passed to this method */ public static void main(String[] args) { System.out.println("Welcome to Four of a Kind!"); Deck deck = new Deck(); // Deck automatically shuffled DiscardPile discardPile = new DiscardPile(); Card hCard; Card cCard; while (true) { hCard = deck.deal(); cCard = deck.deal(); if (hCard.rank() != cCard.rank()) break; CHAPTER 1: Getting Started with Java 31 deck.putBack(hCard); deck.putBack(cCard); deck.shuffle(); // prevent pathological case where every successive } // pair of cards have the same rank int curPlayer = HUMAN; if (cCard.rank().ordinal() > hCard.rank().ordinal()) curPlayer = COMPUTER; deck.putBack(hCard); hCard = null; deck.putBack(cCard); cCard = null; Card[] hCards = new Card[4]; Card[] cCards = new Card[4]; if (curPlayer == HUMAN) for (int i = 0; i < 4; i++) { cCards[i] = deck.deal(); hCards[i] = deck.deal(); } else for (int i = 0; i < 4; i++) { hCards[i] = deck.deal(); cCards[i] = deck.deal(); } while (true) { if (curPlayer == HUMAN) { showHeldCards(hCards); int choice = 0; while (choice < 'A' || choice > 'D') { choice = prompt("Which card do you want to throw away (A, B, " + "C, D)? "); switch (choice) { case 'a': choice = 'A'; break; case 'b': choice = 'B'; break; case 'c': choice = 'C'; break; case 'd': choice = 'D'; } } discardPile.setTopCard(hCards[choice-'A']); hCards[choice-'A'] = deck.deal(); if (isFourOfAKind(hCards)) { System.out.println(); System.out.println("Human wins!"); System.out.println(); putDown("Human's cards:", hCards); System.out.println(); putDown("Computer's cards:", cCards); return; // Exit application by returning from main() } curPlayer = COMPUTER; } CHAPTER 1: Getting Started with Java 32 else { int choice = leastDesirableCard(cCards); discardPile.setTopCard(cCards[choice]); cCards[choice] = deck.deal(); if (isFourOfAKind(cCards)) { System.out.println(); System.out.println("Computer wins!"); System.out.println(); putDown("Computer's cards:", cCards); return; // Exit application by returning from main() } curPlayer = HUMAN; } if (deck.isEmpty()) { while (discardPile.topCard() != null) deck.putBack(discardPile.getTopCard()); deck.shuffle(); } } } /** * Determine if the Card objects passed to this method all * have the same rank. * * @param cards array of Card objects passed to this method * * @return true if all Card objects have the same rank; * otherwise, false */ static boolean isFourOfAKind(Card[] cards) { for (int i = 1; i < cards.length; i++) if (cards[i].rank() != cards[0].rank()) return false; return true; } /** * Identify one of the Card objects that is passed to this * method as the least desirable Card object to hold onto. * * @param cards array of Card objects passed to this method * * @return 0-based rank (ace is 0, king is 13) of least desirable card */ static int leastDesirableCard(Card[] cards) { int[] rankCounts = new int[13]; for (int i = 0; i < cards.length; i++) rankCounts[cards[i].rank().ordinal()]++; int minCount = Integer.MAX_VALUE; int minIndex = -1; for (int i = 0; i < rankCounts.length; i++) if (rankCounts[i] < minCount && rankCounts[i] != 0) { CHAPTER 1: Getting Started with Java 33 minCount = rankCounts[i]; minIndex = i; } for (int i = 0; i < cards.length; i++) if (cards[i].rank().ordinal() == minIndex) return i; return 0; // Needed to satisfy compiler (should never be executed) } /** * Prompt the human player to enter a character. * * @param msg message to be displayed to human player * * @return integer value of character entered by user. */ static int prompt(String msg) { System.out.print(msg); try { int ch = System.in.read(); // Erase all subsequent characters including terminating \n newline // so that they do not affect a subsequent call to prompt(). while (System.in.read() != '\n'); return ch; } catch (java.io.IOException ioe) { } return 0; } /** * Display a message followed by all cards held by player. This output * simulates putting down held cards. * * @param msg message to be displayed to human player * @param cards array of Card objects to be identified */ static void putDown(String msg, Card[] cards) { System.out.println(msg); for (int i = 0; i < cards.length; i++) System.out.println(cards[i]); } /** * Identify the cards being held via their Card objects on * separate lines. Prefix each line with an uppercase letter starting with * A. * * @param cards array of Card objects to be identified */ static void showHeldCards(Card[] cards) { System.out.println(); System.out.println("Held cards:"); for (int i = 0; i < cards.length; i++) if (cards[i] != null) CHAPTER 1: Getting Started with Java 34 System.out.println((char) ('A'+i) + ". " + cards[i]); System.out.println(); } } Listing 1–8 follows the steps outlined by and expands on Listing 1–4’s pseudocode. Because of the various comments, I do not have much to say about this listing. However, there are a couple of items that deserve mention:  Card’s nested Rank enum stores a sequence of 13 Rank objects beginning with ACE and ending with KING. These objects cannot be compared directly via > to determine which object has the greater rank. However, their integer-based ordinal (positional) values can be compared by calling the Rank object’s ordinal() method. For example, Card.ACE_OF_SPADES.rank().ordinal() returns 0 because ACE is located at position 0 within Rank’s list of Rank objects, and Card.KING_OF_CLUBS.rank().ordinal() returns 12 because KING is located at the last position in this list.  The leastDesirableCard() method counts the ranks of the Cards in the array of Card objects passed to this method, and stores these counts in a rankCounts array. For example, given two of clubs, ace of spades, three of clubs, and ace of diamonds, this array identifies one two, two aces, and one three. This method then searches rankCounts from smallest index (representing ace) to largest index (representing king) for the first smallest nonzero count (there might be a tie, as in one two and one three)—a zero count represents no Cards having that rank in the array of Card objects. Finally, the method searches the array of Card objects to identify the object whose rank ordinal matches the index of the smallest nonzero count, and returns the index of this Card object. This behavior implies that the least desirable card is always the smallest ranked card. Furthermore, if there are multiple cards of the same rank, and if this rank is smaller than the rank of another card in the array, this method will choose the first (in a left-to-right manner) of the multiple cards having the same rank as the least desirable card. However, if the rank of the multiple cards is greater than the rank of another card, this other card will be chosen as least desirable. I previously stated that Listing 1–5 begins with a Javadoc comment that describes the Card class and identifies this class’s author. A Javadoc comment is a documentation item that documents a class, a method, or other program entity. A Javadoc comment begins with /** and ends with */. Sandwiched between these delimiters (a pair of characters that mark the start and stop of some section) are text, HTML tags (such as

and ), and Javadoc tags, which are @-prefixed instructions. The following list identifies three common tags: This book was purchased by harshamv@variable3.comCHAPTER 1: Getting Started with Java 35  @author identifies the source code’s author.  @param identifies one of a method’s parameters (discussed in Chapter 2).  @return identifies the kind of value that a method returns. The JDK provides a javadoc tool that extracts all Javadoc comments from one or more source files and generates a set of HTML files containing this documentation in an easy- to-read format. These files serve as the program’s documentation. For example, suppose that the current directory contains Card.java, Deck.java, DiscardPile.java, and FourOfAKind.java. To extract all of the Javadoc comments that appear in these files, specify the following command: javadoc *.java The javadoc tool responds by outputting the following messages: Loading source file Card.java... Loading source file Deck.java... Loading source file DiscardPile.java... Loading source file FourOfAKind.java... Constructing Javadoc information... Standard Doclet version 1.6.0_16 Building tree for all the packages and classes... Generating Card.html... Generating Card.Rank.html... Generating Card.Suit.html... Generating Deck.html... Generating DiscardPile.html... Generating FourOfAKind.html... Generating package-frame.html... Generating package-summary.html... Generating package-tree.html... Generating constant-values.html... Building index for all the packages and classes... Generating overview-tree.html... Generating index-all.html... Generating deprecated-list.html... Building index for all classes... Generating allclasses-frame.html... Generating allclasses-noframe.html... Generating index.html... Generating help-doc.html... Generating stylesheet.css... Furthermore, it generates a series of files, including the index.html entry-point file. If you point your web browser to this file, you should see a page that is similar to the page shown in Figure 1–8. CHAPTER 1: Getting Started with Java 36 Figure 1–8. The entry-point page into the generated Javadoc for FourOfAKind and supporting classes javadoc defaults to generating HTML-based documentation for public classes and public/protected members of classes. You will learn about public classes and public/protected members of classes in Chapter 2. For this reason, FourOfAKind’s documentation reveals only the public main() method. It does not reveal isFourOfAKind() and the other package-private methods. If you want to include these methods in the documentation, you must specify -package with javadoc: javadoc -package *.java NOTE: The standard class library’s documentation was also generated by javadoc and adheres to the same format. CHAPTER 1: Getting Started with Java 37 Compiling, Running, and Distributing FourOfAKind Unlike the previous DumpArgs and EchoText applications, which each consist of one source file, FourOfAKind consists of Card.java, Deck.java, DiscardPile.java, and FourOfAKind.java. You can compile all four source files via the following command line: javac FourOfAKind.java The javac tool launches the Java compiler, which recursively compiles the source files of the various classes it encounters during compilation. Assuming successful compilation, you should end up with six classfiles in the current directory. TIP: You can compile all Java source files in the current directory by specifying javac *.java. After successfully compiling FourOfAKind.java and the other three source files, specify the following command line to run this application: java FourOfAKind In response, you see an introductory message and the four cards that you are holding. The following output reveals a single session: Welcome to Four of a Kind! Held cards: A. SIX_OF_CLUBS B. QUEEN_OF_DIAMONDS C. SIX_OF_HEARTS D. SIX_OF_SPADES Which card do you want to throw away (A, B, C, D)? B Held cards: A. SIX_OF_CLUBS B. NINE_OF_HEARTS C. SIX_OF_HEARTS D. SIX_OF_SPADES Which card do you want to throw away (A, B, C, D)? B Held cards: A. SIX_OF_CLUBS B. FOUR_OF_DIAMONDS C. SIX_OF_HEARTS D. SIX_OF_SPADES Which card do you want to throw away (A, B, C, D)? B Held cards: A. SIX_OF_CLUBS B. KING_OF_HEARTS C. SIX_OF_HEARTS D. SIX_OF_SPADES CHAPTER 1: Getting Started with Java 38 Which card do you want to throw away (A, B, C, D)? B Held cards: A. SIX_OF_CLUBS B. QUEEN_OF_CLUBS C. SIX_OF_HEARTS D. SIX_OF_SPADES Which card do you want to throw away (A, B, C, D)? B Held cards: A. SIX_OF_CLUBS B. KING_OF_DIAMONDS C. SIX_OF_HEARTS D. SIX_OF_SPADES Which card do you want to throw away (A, B, C, D)? B Held cards: A. SIX_OF_CLUBS B. TWO_OF_HEARTS C. SIX_OF_HEARTS D. SIX_OF_SPADES Which card do you want to throw away (A, B, C, D)? B Held cards: A. SIX_OF_CLUBS B. FIVE_OF_DIAMONDS C. SIX_OF_HEARTS D. SIX_OF_SPADES Which card do you want to throw away (A, B, C, D)? B Held cards: A. SIX_OF_CLUBS B. JACK_OF_CLUBS C. SIX_OF_HEARTS D. SIX_OF_SPADES Which card do you want to throw away (A, B, C, D)? B Held cards: A. SIX_OF_CLUBS B. TWO_OF_SPADES C. SIX_OF_HEARTS D. SIX_OF_SPADES Which card do you want to throw away (A, B, C, D)? B Human wins! Human's cards: SIX_OF_CLUBS SIX_OF_DIAMONDS SIX_OF_HEARTS CHAPTER 1: Getting Started with Java 39 SIX_OF_SPADES Computer's cards: SEVEN_OF_HEARTS TEN_OF_HEARTS SEVEN_OF_CLUBS SEVEN_OF_DIAMONDS Although Four of a Kind is not much of a card game, you might decide to share the FourOfAKind application with a friend. However, if you forget to include even one of the application’s five supporting classfiles, your friend will not be able to run the application. You can overcome this problem by bundling FourOfAKind’s six classfiles into a single JAR (Java ARchive) file, which is a ZIP file that contains a special directory and the .jar file extension. You can then distribute this single JAR file to your friend. The JDK provides the jar tool for working with JAR files. To bundle all six classfiles into a JAR file named FourOfAKind.jar, you could specify the following command line, where c tells jar to create a JAR file and f identifies the JAR file’s name: jar cf FourOfAKind.jar *.class After creating the JAR file, try to run the application via the following command line: java -jar FourOfAKind.jar Instead of the application running, you will receive an error message having to do with java not knowing which of the JAR file’s six classfiles is the main classfile (the file whose class’s main() method executes first). You can provide this knowledge via a text file that is merged into the JAR file’s manifest, a special file named MANIFEST.MF that stores information about the contents of a JAR file, and which is stored in the JAR file’s META-INF directory. Consider Listing 1–9. Listing 1–9. Identifying the application’s main class Main-Class: FourOfAKind Listing 1–9 tells java which of the JAR’s classfiles is the main class file. (You must leave a blank line after Main-Class: FourOfAKind.) The following command line, which creates FourOfAKind.jar, includes m and the name of the text file providing manifest content: jar cfm FourOfAKind.jar manifest *.class This time, java -jar FourOfAKind.jar succeeds and the application runs because java is able to identify FourOfAKind as the main classfile. CHAPTER 1: Getting Started with Java 40 EXERCISES The following exercises are designed to test your understanding of what Java means, the JDK, NetBeans, Eclipse, and Java application development: 1. What is Java? 2. What is a virtual machine? 3. What is the purpose of the Java compiler? 4. True or false: A classfile’s instructions are commonly referred to as bytecode. 5. What does the virtual machine’s interpreter do when it learns that a sequence of bytecode instructions is being executed repeatedly? 6. How does the Java platform promote portability? 7. How does the Java platform promote security? 8. True or false: Java SE is the Java platform for developing servlets. 9. What is the JRE? 10. What is the difference between the public and private JREs? 11. What is the JDK? 12. Which JDK tool is used to compile Java source code? 13. Which JDK tool is used to run Java applications? 14. What is the purpose of the JDK’s jar tool? 15. What is Standard I/O? 16. What is an IDE? 17. Identify two popular IDEs. 18. What is pseudocode? 19. How would you list FourOfAKind.jar’s table of contents (list of directories and files contained in the JAR file)? 20. Modify FourOfAKind to give each player the option of picking up the top card from the deck or discard pile (assuming that the discard pile is not empty; it is empty when the human player goes first and starts his/her first turn). Display the discard pile’s top card for the human player’s benefit. (Do not display the deck’s top card.) CHAPTER 1: Getting Started with Java 41 Summary Java is a language and a platform. The language is partly patterned after the C and C++ languages to shorten the learning curve for C/C++ developers. The platform consists of a virtual machine and associated execution environment. Developers use different editions of the Java platform to create Java programs that run on desktop computers, web browsers, web servers, mobile information devices, and embedded devices. These editions are known as Java SE, Java EE, and Java ME. Developers also use a special Google-created edition of the Java platform to create Android apps that run on Android-enabled devices. This edition, known as the Android platform, largely consists of Java core libraries and a virtual machine known as Dalvik. The public JRE implements the Java SE platform and makes it possible to run Java programs. The JDK provides tools (including the Java compiler) for developing Java programs and also includes a private copy of the JRE. Working with the JDK’s tools at the command line is not recommended for large projects, which are hard to manage without the help of an integrated development environment. Two popular IDEs are NetBeans and Eclipse. Application development is not an easy task. All applications except for the most trivial require careful planning or you will probably waste your (and your users’) time and money. One way to develop applications efficiently involves using pseudocode. FourOfAKind gave you a significant taste of the Java language. Although much of its source code is probably hard to understand right now, you will find it much easier to grasp after reading Chapter 2, which introduces you to Java’s language fundamentals. CHAPTER 1: Getting Started with Java 42 43 43 Chapter Learning Language Fundamentals Aspiring Android developers need to understand the Java language. Java is an object- oriented language in which developers use objects to represent entities (things that exist, such as vehicles, checking and savings bank accounts, and buttons and other user interface components). Language features supporting this paradigm are the focus of this chapter and Chapter 3. More advanced features are discussed in Chapters 4 and 5. Additional advanced but minor features are more appropriately discussed in later chapters. Classes Object-oriented applications represent entities as objects (entity abstractions). Each object encapsulates (combines into a single unit) an entity’s attributes and behaviors. For example, a checking bank account object might encapsulate a balance attribute (with a current value of $50) along with deposit and withdrawal behaviors. NOTE: Encapsulation exists in stark contrast to the separation of attributes and behaviors in C and other structured programming languages. In such a language, the developer cannot merge an entity’s attributes and behaviors into an object. Instead, the developer must separately declare attributes via suitable data structures (organizations of data) and behaviors via suitable functions, to which data structure instances that contain attribute values are passed. Objects do not pop out of thin air; they must be instantiated (created) from something. Languages such as C++ and Java refer to this something as a class, a template for manufacturing objects (also known as class instances, or instances for short). This section introduces you to Java’s language features for architecting classes. 2 CHAPTER 2: Learning Language Fundamentals 44 Declaring Classes Because you cannot instantiate objects from a class that does not exist, you must declare the class. The declaration consists of a header followed by a body. At minimum, the header consists of reserved word class followed by a name that identifies the class (so that it can be referred to from elsewhere in the source code). The body starts with an open brace character ({) and ends with a close brace (}). Sandwiched between these delimiters (a pair of characters that mark the start and stop of some section) are field, method, and other kinds of declarations. Consider Listing 2–1. Listing 2–1. Declaring a skeletal CheckingAccount class class CheckingAccount { // field, method, and other member declarations } Listing 2–1 declares a class named CheckingAccount. By convention, a class’s name begins with an uppercase letter. Furthermore, the first letter of each subsequent word in a multiword class name is capitalized. This is known as camel-casing. NOTE: A class declaration is an example of type, a template for a set of data values and the operations that can be legally performed on these values. CheckingAccount is one example of an identifier, which is a name that identifies a class or other source code entity. Identifiers consist of letters (A-Z, a-z, or equivalent uppercase/lowercase letters in other human languages), digits (0-9 or equivalent digits in other human languages), connecting punctuation characters (such as the underscore), and currency symbols (such as the dollar sign). However, an identifier can only begin with a letter, a currency symbol, or a connecting punctuation character. Also, an identifier’s maximum length cannot exceed the length of the line in which it appears. NOTE: Identifiers are expressed in Unicode (http://en.wikipedia.org/wiki/Unicode), a universal character set that encodes the various symbols making up the world’s written languages. Additional examples include temperature, Temperature, _class, first$name, and loopCounter1. temperature and Temperature are two different identifiers because Java is a case-sensitive language. In contrast, 6x and door^color are not identifiers because the former character sequence begins with a digit, and the latter character sequence contains an illegal character (^). You can choose almost any identifier to name classes and other source code entities. However, Java reserves the following identifiers, which are commonly referred to as reserved words, for special uses: abstract, assert, boolean, break, byte, case, catch, char, class, const, continue, default, do, double, enum, else, extends, false, final, finally, float, for, goto, if, implements, import, instanceof, int, interface, long, This book was purchased by harshamv@variable3.comCHAPTER 2: Learning Language Fundamentals 45 native, new, null, package, private, protected, public, return, short, static, strictfp, super, switch, synchronized, this, throw, throws, transient, true, try, void, volatile, and while. Attempting to use any of these reserved words outside of their usage contexts results in compiler error messages. The class body is currently empty because we have yet to explore fields, methods, and other class members. This fact is pointed out by the single-line comment, a line of documentation that begins with //. The compiler ignores everything from // to the end of the line. Introducing Fields After declaring a class, you can declare variables (memory locations whose values can change) in the class’s body. Some of these variables are used in an object context to describe entity attributes. Other variables are used in a class context to describe class attributes (attributes that are shared by all created objects, such as a variable that contains a count of all objects created from the class). Regardless of its purpose, a variable that is introduced into a class’s body is known as a field. This section shows you how to declare fields, how to initialize them to nondefault values, and how to declare read-only fields. Declaring Fields You can declare a field within a class’s body by minimally specifying a type name, followed by an identifier that names the field, followed by a semicolon character (;). Listing 2–2 presents a pair of field declarations. Listing 2–2. Declaring owner and balance fields in the CheckingAccount class class CheckingAccount { String owner; // name of person who owns this checking account int balance; // number of dollars that can be withdrawn } Listing 2–2 declares two fields named owner and balance. By convention, a field’s name begins with a lowercase letter, and the first letter of each subsequent word in a multiword field name is capitalized. A field’s type identifies the kind of values that can be assigned to the field. The owner field is of type String, which is one of Java’s predefined classes. String objects contain sequences of characters; the character sequence in any String object assigned to owner will contain the name of the account’s owner. The balance field is of type integer, which is implied by reserved word int. (In common practice, we say that balance is of type int.) This field can store the account balance as a whole number without a fraction. CHAPTER 2: Learning Language Fundamentals 46 Integer is one of several primitive types (types whose values are not objects) that are supported by Java. Table 2–1 describes all of Java’s primitive types except for void (discussed later). Table 2–1. Primitive Types Primitive Type Reserved Word Size Min Value Max Value Boolean boolean -- -- -- Character char 16-bit Unicode 0 Unicode 216 - 1 Byte integer byte 8-bit -128 +127 Short integer short 16-bit -215 +215 - 1 Integer int 32–bit -231 +231 - 1 Long integer long 64-bit -263 +263 - 1 Floating-point float 32–bit IEEE 754 IEEE 754 Double precision floating-point double 64-bit IEEE 754 IEEE 754 Table 2–1 expresses primitive type sizes in terms of the number of bits (binary digits— each digit is either 0 or 1) that a value of that type occupies in memory. A group of eight bits is known as a byte. Also, Unicode 0 is shorthand for “the first Unicode character,” and IEEE 754 (http://en.wikipedia.org/wiki/IEEE_754) refers to a standard for representing floating- point numbers in memory. Except for Boolean, whose size is implementation dependent (one Java implementation might store a Boolean value in a single bit, whereas another implementation might require an eight-bit byte for performance efficiency), each primitive type’s implementation has a specific size. NOTE: Unlike their C and C++ counterparts, Java’s primitive types have the same size in each Java implementation, which partly accounts for the portability of Java applications. Except for Boolean, whose only values are true and false, each primitive type has a minimum and a maximum value. By studying these limits, you can deduce that the character type is unsigned (all values are positive). In contrast, each numeric type is signed (the type supports positive and negative values). CHAPTER 2: Learning Language Fundamentals 47 NOTE: Developers who argue that everything in Java should be an object are not happy about the inclusion of primitive types in the language. However, Java was designed to include primitive types to overcome the speed and memory limitations of early 1990s-era devices, to which Java was originally targeted. The owner and balance fields are examples of non-array fields because each field can hold only one value—an array is a multivalue variable; each array element (storage slot) holds one of these values. Java also lets you declare array-based fields, which, as Listing 2–3 reveals, are identified by the presence of square brackets ([ and ]). Listing 2–3. Declaring cities and temperatures array-based fields in a WeatherData class class WeatherData { String country; String[] cities; double[][]temperatures; } Listing 2–3 declares a WeatherData class that holds temperature extremes for a variety of cities in a specific country. The one-dimensional cities array contains the names of these cities; the two-dimensional temperatures array contains the maximum and minimum temperature values for each city. A one-dimensional array is a sequential list of values; it is identified in source code by one pair of square brackets. A two-dimensional array is a table of values; it is identified in source code by two pairs of square brackets. These brackets can appear on either side of the field name, but are often shown with the type name. (Java also supports higher-dimensional arrays.) CheckingAccount’s owner and balance fields, and WeatherData’s country, cities, and temperatures fields are examples of instance fields because they associate with objects. Each CheckingAccount instance is given its own copy of owner and balance when the object is created. Similarly, each WeatherData instance is given its own copy of country, cities, and temperatures when the object is created. Modifying any field’s value does not affect the value in any other copy of that field. In many situations, instance fields are all that you need. However, you might encounter a situation where you need a single copy of a field no matter how many objects are created. For example, suppose you want to track the number of CheckingAccount objects that have been created, and introduce a counter field (initialized to 0) into this class. You also place code in the class’s constructor (which I will present when I discuss constructors) to increase counter’s value by 1 when an object is created. However, because each object has its own copy of the counter field, this field’s value never advances past 1. CHAPTER 2: Learning Language Fundamentals 48 You can solve this problem by declaring counter to be a class field, a field that associates with a class instead of with that class’s objects. Listing 2–4 accomplishes this task by prefixing counter’s declaration with the static reserved word. Listing 2–4. Adding a counter class field to CheckingAccount class CheckingAccount { String owner; int balance; static int counter; } Listing 2–4’s static prefix implies that there is only one copy of the counter field, not one copy per object. Each time an object is created, counter will increase by 1, and you will get an accurate tally of all CheckingAccount objects that have been created. Each of owner and balance is created when a CheckingAccount object is created, and destroyed when the object is destroyed. In contrast, counter is created when CheckingAccount is loaded into memory, and destroyed when this class is removed from memory (when the application ends). This quality is known as lifetime. Each of owner and balance can be accessed only from an instance context (such as a constructor). In contrast, counter can be accessed from instance and class contexts (such as a class method, discussed later in this chapter). This quality is known as scope. Initializing Fields Listing 2–4’s owner and balance fields are initialized to default null and 0 values (respectively) when an object is created from the CheckingAccount class. The counter field is initialized to 0 when this class is loaded into memory. It is common to initialize an instance field to a (potentially unique) value from within the class’s constructor. However, you might want to explicitly assign some reasonable nonunique default value to this field, and give yourself the option of overriding that default value via the constructor. In contrast, you often explicitly initialize class fields. Regardless of how you initialize a field, the value results from evaluating an expression, which is a combination of variables, method calls (invocations), literals (values specified verbatim), and operators. The expression’s type must agree with the field’s type when assigning the expression to the field. Otherwise, the compiler reports an error. Simple Expressions A simple expression is a value expressed as a variable name (value is read from the variable), a method call (value is returned from the method), or a literal in source code. Java supports several kinds of literals: string, Boolean, character, integer, floating-point, and null (which I will discuss later in this chapter). A string literal consists of a sequence of Unicode characters surrounded by a pair of double quotes ("). Example: "The quick brown fox jumps over the lazy dog." CHAPTER 2: Learning Language Fundamentals 49 A string literal might also contain escape sequences, which are special syntax (rules for combining symbols into language features) for representing certain printable and nonprintable characters that otherwise cannot appear in the literal. For example, string literal "The quick brown \"fox\" jumps over the lazy dog." uses the \" escape sequence to surround fox with double quotes. Table 2–2 describes all supported escape sequences. Table 2–2. Escape Sequences Escape Syntax Description \\ Backslash \" Double quote \' Single quote \b Backspace \f Form feed \n Newline (also referred to as linefeed) \r Carriage return \t Horizontal tab Finally, a string literal might contain Unicode escape sequences, which are special syntax for representing Unicode characters. A Unicode escape sequence begins with \u and continues with four hexadecimal digits (0-9, A-F, a-f) with no intervening space. For example, \u0041 represents capital letter A, and \u20ac represents the European Union’s euro currency symbol. A Boolean literal consists of reserved word true or reserved word false. A character literal consists of a single Unicode character surrounded by a pair of single quotes ('A' is an example). You can also represent, as a character literal, an escape sequence ('\'', for example) or a Unicode escape sequence (such as '\u0041'). An integer literal consists of a sequence of digits. If the literal is to represent a long integer value, then it must be suffixed with an uppercase L or lowercase l (L is easier to read). If there is no suffix, the literal represents a 32–bit integer (an int). Integer literals can be specified in the default decimal, hexadecimal, octal, and (starting with Java version 7) binary formats:  The decimal format is the default format. Example: 255  The hexadecimal format requires that the literal be prefixed with 0x or 0X and continue with hexadecimal digits (0-9, A-F, a-f). Example: 0xFF CHAPTER 2: Learning Language Fundamentals 50  The octal format requires that the literal be prefixed with 0 and continue with octal digits (0-7). Example: 077  The binary format requires that the literal be prefixed with 0b or 0B and continue with 0s and 1s. Example: 0b11111111 Java version 7 also adds another nice feature where integer literals are concerned: it lets you insert underscores between digits to improve readability. Example: 123_456_789. Although you can insert multiple successive underscores between digits (as in 0b1111__0000), you cannot specify a leading underscore (as in _123) because the compiler would treat the literal as an identifier. Also, you cannot specify a trailing underscore (as in 123_). Finally, a floating-point literal consists of an integer part, a decimal point (represented by the period character (.)), a fractional part, an exponent (starting with letter E or e), and a type suffix (letter D, d, F, or f). Most parts are optional, but enough information must be present to differentiate the floating-point literal from an integer literal. Examples include 0.1 (double precision floating-point), 10F (floating-point), 10D (double precision floating- point), and 3.0E+23 (double precision floating-point). Suppose you need to create a class whose objects are used to log messages to a specific file. Each instance will have a field that identifies the target file, initialized to a default filename. Listing 2–5 presents this class and shows you how to initialize this field. Listing 2–5. Initializing the Logger class's filename field class Logger { String filename = "log.txt"; } Listing 2–5 uses simple initialization to initialize a non-array field. You can also use simple initialization with array-based fields. For example, Listing 2–6 shows you how to initialize array-based cities and temperatures fields. Listing 2–6. Initializing the WeatherData class's fields class WeatherData { String country = "United States"; String[] cities = {"Chicago", "New York", "Los Angeles"}; double[][] temperatures = {{0.0, 0.0}, {0.0, 0.0}, {0.0, 0.0}}; } Listing 2–6’s WeatherData class defaults its objects to providing weather data for three American cities. The cities array is initialized to a comma-separated list of three String literals, which are specified between braces; the temperatures array is initialized to a three-row by two-column table of double precision floating-point zeros, which is specified as a three-element row array with each comma-separated element containing a two-element column array. CHAPTER 2: Learning Language Fundamentals 51 Compound Expressions A compound expression is a sequence of simple expressions and operators, where an operator is a sequence of instructions (symbolically represented in source code) that transforms one or more values, known as operands, into another value. For example, the + symbol denotes the addition or string concatenation operator, depending on the types of its two operands. When this symbol appears between numeric operands (6+4, for example), addition is implied. Similarly, when + appears between string literals (as in "A"+"B"), string concatenation is implied. Java supplies a wide variety of operators that are classified by the number of operands they take. A unary operator takes only one operand, a binary operator takes two operands, and Java’s single ternary operator takes three operands. These operators are also classified as prefix, postfix, and infix. A prefix operator is a unary operator that precedes its operand, a postfix operator is a unary operator that trails its operand, and an infix operator is a binary or ternary operator that is sandwiched between the binary operator’s two or the ternary operator’s three operands. Table 2–3 describes all supported operators. (I will explain precedence after this table.) Table 2–3. Operators Operator Symbol Description Precedence Addition + Given operand1 + operand2, where each operand must be of character or numeric type, add operand2 to operand1 and return the sum. 10 Array index [] Given variable[index], where index must be of integral type, read value from or store value into variable’s storage element at location index. 13 Assignment = Given variable = operand, which must be assignment-compatible (their types must agree), store operand in variable. 0 Bitwise AND & Given operand1 & operand2, where each operand must be of character or integer type, bitwise AND their corresponding bits and return the result. A result bit is set to 1 if each operand’s corresponding bit is 1. Otherwise, the result bit is set to 0. 6 Bitwise complement ~ Given ~operand, where operand must be of character or integer type, flip operand’s bits (1s to 0s and 0s to 1s) and return the result. 12 Bitwise exclusive OR ^ Given operand1 ^ operand2, where each operand must be of character or integer type, bitwise exclusive OR their corresponding bits and return the result. A result bit is set to 1 if one operand’s corresponding bit is 1 and the other operand’s corresponding bit is 0. Otherwise, the result bit is set to 0. 5 CHAPTER 2: Learning Language Fundamentals 52 Operator Symbol Description Precedence Bitwise inclusive OR | Given operand1 | operand2, which must be of character or integer type, bitwise inclusive OR their corresponding bits and return the result. A result bit is set to 1 if either (or both) of the operands’ corresponding bits is 1. Otherwise, the result bit is set to 0. 4 Cast (type) Given (type) operand, convert operand to an equivalent value that can be represented by type. For example, you could use this operator to convert a floating-point value to a 32–bit integer value. 12 Compound assignment +=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>=, >>>= Given variable operator operand, where operator is one of the listed compound operator symbols, and where operand is assignment-compatible with variable, perform the indicated operation using variable’s value as operator’s left operand value, and store the resulting value in variable. 0 Conditional ?: Given operand1 ? operand2 : operand3, where operand1 must be of Boolean type, return operand2 if operand1 is true or operand3 if operand1 is false. The types of operand2 and operand3 must agree. 1 Conditional AND && Given operand1 && operand2, where each operand must be of Boolean type, return true if both operands are true. Otherwise, return false. If operand1 is false, operand2 is not examined. This is known as short- circuiting. 3 Conditional OR || Given operand1 || operand2, where each operand must be of Boolean type, return true if at least one operand is true. Otherwise, return false. If operand1 is true, operand2 is not examined. This is known as short- circuiting. 2 Division / Given operand1 / operand2, where each operand must be of character or numeric type, divide operand1 by operand2 and return the quotient. 11 Equality == Given operand1 == operand2, where both operands must be comparable (you cannot compare an integer with a string literal, for example), compare both operands for equality. Return true if these operands are equal. Otherwise, return false. 7 Inequality != Given operand1 != operand2, where both operands must be comparable (you cannot compare an integer with a string literal, for example), compare both operands for inequality. Return true if these operands are not equal. Otherwise, return false. 7 CHAPTER 2: Learning Language Fundamentals 53 Operator Symbol Description Precedence Left shift << Given operand1 << operand2, where each operand must be of character or integer type, shift operand1’s binary representation left by the number of bits that operand2 specifies. For each shift, a 0 is shifted into the rightmost bit and the leftmost bit is discarded. Only the five low-order bits of operand2 are used when shifting a 32–bit integer (to prevent shifting more than the number of bits in a 32–bit integer). Only the six low- order bits of operand2 are used when shifting a 64-bit integer (to prevent shifting more than the number of bits in a 64-bit integer). The shift preserves negative values. Furthermore, it is equivalent to (but faster than) multiplying by a multiple of 2. 9 Logical AND & Given operand1 & operand2, where each operand must be of Boolean type, return true if both operands are true. Otherwise, return false. In contrast to conditional AND, logical AND does not perform short-circuiting. 6 Logical complement ! Given !operand, where operand must be of Boolean type, flip operand’s value (true to false or false to true) and return the result. 12 Logical exclusive OR ^ Given operand1 ^ operand2, where each operand must be of Boolean type, return true if one operand is true and the other operand is false. Otherwise, return false. 5 Logical inclusive OR | Given operand1 | operand2, where each operand must be of Boolean type, return true if at least one operand is true. Otherwise, return false. In contrast to conditional OR, logical inclusive OR does not perform short-circuiting. 4 Member access . Given identifier1.identifier2, access the identifier2 member of identifer1. You will learn about this operator later in this chapter. 13 Method call () Given identifier(argument list), call the method identified by identifier and matching parameter list. You will learn about calling methods later in this chapter. 13 Multiplication * Given operand1 * operand2, where each operand must be of character or numeric type, multiply operand1 by operand2 and return the product. 11 Object creation new Given new identifier(argument list), allocate memory for object and call constructor specified as identifier(argument list). Given new identifier[integer size], allocate a one-dimensional array of values. 12 Postdecrement -- Given variable--, where variable must be of character or numeric type, subtract 1 from variable’s value (storing the result in variable) and return the original value. 13 CHAPTER 2: Learning Language Fundamentals 54 Operator Symbol Description Precedence Postincrement ++ Given variable++, where variable must be of character or numeric type, add 1 to variable’s value (storing the result in variable) and return the original value. 13 Predecrement -- Given --variable, where variable must be of character or numeric type, subtract 1 from its value, store the result in variable, and return this value. 12 Preincrement ++ Given ++variable, where variable must be of character or numeric type, add 1 to its value, store the result in variable, and return this value. 12 Relational greater than > Given operand1 > operand2, where each operand must be of character or numeric type, return true if operand1 is greater than operand2. Otherwise, return false. 8 Relational greater than or equal to >= Given operand1 >= operand2, where each operand must be of character or numeric type, return true if operand1 is greater than or equal to operand2. Otherwise, return false. 8 Relational less than < Given operand1 < operand2, where each operand must be of character or numeric type, return true if operand1 is less than operand2. Otherwise, return false. 8 Relational less than or equal to <= Given operand1 <= operand2, where each operand must be of character or numeric type, return true if operand1 is less than or equal to operand2. Otherwise, return false. 8 Relational type checking instanceof Given operand1 instanceof operand2, where operand1 is an object and operand2 is a class (or other user- defined type), return true if operand1 is an instance of operand2. Otherwise, return false. 8 Remainder % Given operand1 % operand2, where each operand must be of character or numeric type, divide operand1 by operand2 and return the remainder. 11 Signed right shift >> Given operand1 >> operand2, where each operand must be of character or integer type, shift operand1’s binary representation right by the number of bits that operand2 specifies. For each shift, a copy of the sign bit (the leftmost bit) is shifted to the right and the rightmost bit is discarded. Only the five low-order bits of operand2 are used when shifting a 32–bit integer (to prevent shifting more than the number of bits in a 32–bit integer). Only the six low-order bits of operand2 are used when shifting a 64-bit integer (to prevent shifting more than the number of bits in a 64-bit integer). The shift preserves negative values. Furthermore, it is equivalent to (but faster than) dividing by a multiple of 2. 9 This book was purchased by harshamv@variable3.comCHAPTER 2: Learning Language Fundamentals 55 Operator Symbol Description Precedence String concatenation + Given operand1 + operand2, where at least one operand is of String type, append operand2’s string representation to operand1’s string representation and return the concatenated result. 10 Subtraction - Given operand1 - operand2, where each operand must be of character or numeric type, subtract operand2 from operand1 and return the difference. 10 Unary minus - Given -operand, where operand must be of character or numeric type, return operand’s arithmetic negative. 12 Unary plus + Like its predecessor, but return operand. Rarely used. 12 Unsigned right shift >>> Given operand1 >>> operand2, where each operand must be of character or integer type, shift operand1’s binary representation right by the number of bits that operand2 specifies. For each shift, a zero is shifted into the leftmost bit and the rightmost bit is discarded. Only the five low-order bits of operand2 are used when shifting a 32–bit integer (to prevent shifting more than the number of bits in a 32–bit integer). Only the six low- order bits of operand2 are used when shifting a 64-bit integer (to prevent shifting more than the number of bits in a 64-bit integer). The shift does not preserve negative values. Furthermore, it is equivalent to (but faster than) dividing by a multiple of 2. 9 In Table 2–3’s operator descriptions, “integer type” refers to any of the byte integer, short integer, integer, or long integer types, unless integer type is qualified as a 32–bit integer. Also, “numeric type” refers to any of these integer types along with floating- point and double precision floating-point. Table 2–3’s rightmost column presents a value that indicates the operator’s precedence (level of importance): the higher the number, the higher the precedence. For example, addition’s precedence level is 10 and multiplication’s precedence level is 11, which means that multiplication is performed before addition when evaluating 40+2*4. Precedence can be circumvented by introducing open and close parentheses, ( and ), into the expression, where the innermost pair of nested parentheses is evaluated first. For example, (40+2)*4 results in addition being performed before multiplication, and 40/(2–4) results in subtraction being performed before divison. During evaluation, operators with the same precedence level (such as addition and subtraction, which both have level 10) are processed according to their associativity (a property that determines how operators having the same precedence are grouped when parentheses are missing). For example, expression 6*3/2 is evaluated as if it was (6*3)/2 because * and / are left- to-right associative operators. In contrast, expression a=b=c=10 is evaluated as if it was a=(b=(c=10))—10 is assigned to c, c’s new value (10) is assigned to b, and b’s new value (10) is assigned to a—because = is a right-to-left associative operator. CHAPTER 2: Learning Language Fundamentals 56 Most of Java’s operators are left-to-right associative. Right-to-left associative operators include assignment, bitwise complement, cast, compound assignment, conditional, logical complement, object creation, predecrement, preincrement, unary minus, and unary plus. Whenever an operator encounters two operands of different types, it attempts to convert one (and possibly both) of the operands to a type that is suitable for performing its operation. For example, if you attempt to add a 16-bit short integer value to a 32–bit integer value, the addition operator will first convert the short integer value to a 32–bit integer value, and then add together both 32–bit integer values. Similarly, if you attempt to add a 32–bit integer value to a 32–bit floating-point value, the operator will first convert the 32–bit integer value to a 32–bit floating-point value, and then add together both floating-point values. The operator always converts the value with a more limited representation (such as a 16-bit short integer) to an equivalent value in a less limited representation (such as a 32–bit integer). Sometimes, you need to explicitly perform a conversion, and this is where the cast operator comes into play. For example, suppose your class contains the following field declarations: char c = 'A'; byte b = c; The compiler reports an error about loss of precision when it encounters byte b = c;. The reason is that c can represent any unsigned integer value from 0 through 65535, whereas b can only represent a signed integer value from -128 through +127. Even though 'A' equates to +65, which can fit within b’s range, c could just have easily been initialized to '\u0123', which would not fit. Because of the potential for data loss, the compiler complains. The solution to this problem involves introducing a (byte) cast, which explicitly tells the compiler that the developer is aware of the potential for data loss but wants the conversion to occur: byte b = (byte) c; Not all conversions can be performed. For example, the subtraction operator cannot subtract a string literal from a 32–bit integer. Also, attempting to cast a floating-point value to a String, as in String s = (String) 20.0;, does not work. In these situations, the compiler provides suitable error messages. The mathematical operators (+, -, *, /, %) can yield values that overflow or underflow the limits of the resulting value’s type. For example, multiplying two large positive 32–bit integer values can produce a value that cannot be represented as a 32–bit integer value. Java does not detect overflows and underflows. Dividing (via / or %) a numeric value by 0 also results in interesting behavior. In the first case, dividing an integer value by integer 0 causes the operator to throw an ArithmeticException object (I briefly discuss ArithmeticException in Chapter 4 when I cover exceptions). Secondly, dividing a floating-point value by 0 causes the operator to CHAPTER 2: Learning Language Fundamentals 57 return +infinity or -infinity, depending on whether the dividend is positive or negative. Finally, dividing floating-point 0 by 0 causes the operator to return NaN (Not a Number). Suppose you have created a ReportWriter class that outputs reports to the printer. During the testing phase, you want to make sure that reports are properly generated without wasting paper, so you would like the output to be directed to a file. An additional requirement is that you would like to be able to change the file’s name or the printer’s name from application code. Listing 2–7 presents a possible solution. Listing 2–7. Initializing the ReportWriter class's outputDevice field with the help of the conditional operator class ReportWriter { static boolean test = true; static String outputDevice = (test) ? "file" : "printer"; } Listing 2–7’s ReportWriter class satisfies the first requirement by providing a test class field. When this class loads, true is assigned to test, overriding its default false value. The expression assigned to the outputDevice class field uses the conditional operator to examine test. Finding this field to contain true, the operator subsequently returns string literal "file", which is assigned to outputDevice. If test is found to contain false, "printer" is returned and assigned to outputDevice. To satisfy the second requirement, simply assign the appropriate destination string literal to outputDevice from the application’s code. Read-only Fields Java provides reserved word final for declaring that a field is read-only. Each object receives its own copy of a read-only instance field. This field must be initialized, as part of the field’s declaration or in the class’s constructor. If initialized in the constructor, the read-only instance field is known as a blank final because it does not have a value until one is assigned to it in the constructor. Because a constructor can potentially assign a different value to each object’s blank final, these read-only variables are not truly constants. If you want a true constant, which is a single read-only value that is available to all objects, you need to create a read-only class field. You can accomplish this task by including the reserved word static with final in that field’s declaration. Listing 2–8 shows you how to declare a read-only class field. Listing 2–8. Declaring a true constant in the Employee class class Employee { final static int RETIREMENT_AGE = 65; } Listing 2–8’s RETIREMENT_AGE declaration is an example of a compile-time constant. Because there is only one copy of its value (thanks to the static reserved word), and CHAPTER 2: Learning Language Fundamentals 58 because this value will never change (thanks to the final reserved word), the compiler is free to optimize the compiled code by inserting the constant value into all calculations where it is used. The code runs faster because it does not have to access a read-only class field. Introducing Methods After declaring a class, you can introduce methods (named bodies of code) into the class’s body. Some of these methods can associate with objects in order to describe entity behaviors. Other methods can associate with classes. This section shows you how to declare methods, how to implement their bodies, and how to overload methods. Declaring Methods You can declare a method within a class’s body by minimally specifying a return type, followed by an identifier that names the method, followed by a parameter list, followed by a body. Listing 2–9 presents a simple method declaration. Listing 2–9. Declaring a printBalance() method in the CheckingAccount class class CheckingAccount { String owner; int balance; static int counter; void printBalance() { // code that outputs the balance field's value } } Listing 2–9 declares a method named printBalance in CheckingAccount’s body. By convention, a method’s name begins with a lowercase letter. Furthermore, the first letter of each subsequent word in a multiword method name is capitalized. A method’s parameter list identifies the number, order, and types of values that are passed to the method when the method is called via a parentheses-delimited and comma-separated list of variable declarations. Each value passed to the method is called an argument; the variable that receives the argument is called a parameter. Because arguments are not passed to printBalance(), its parameter list is empty. NOTE: The method’s name and the types of its parameters are known as its signature. The method’s body specifies code that is to be executed when the method is called. In this example, I chose to leave the body empty, but have supplied a single-line comment to describe what the body should contain. Finally, a method’s return type identifies the kind of values returned by the method. Because printBalance() does not return a value, its return type is set to void via CHAPTER 2: Learning Language Fundamentals 59 reserved word void. (Think of void as a special kind of primitive type, even though variables cannot be declared to be void, which is why I did not include it in Table 2–1.) Because a CheckingAccount class should also provide methods for making deposits and withdrawals, Listing 2–10 introduces deposit() and withdraw() methods into this class. Listing 2–10. Declaring deposit() and withdraw() methods class CheckingAccount { String owner; int balance; static int counter; void printBalance() { // code that outputs the balance field’s value } int deposit(int amount) { // code that adds the specified amount to balance, and returns the new balance } int withdraw(int amount) { /* code that subtracts the specified amount from balance, and returns the new balance */ } } Listing 2–10’s deposit() and withdraw() methods each specify a nonempty parameter list consisting of a single amount parameter. As with any parameter, amount’s lifetime ranges from the point where execution enters the method to the point where execution exits the method and returns to the method’s caller (the code that called the method). Also, amount’s scope is the entire method. The deposit() and withdraw() methods also specify, via their int return types, that they return integer values. Because we are not yet ready to code these methods’ bodies, I have introduced comments to document their tasks. In contrast to printBalance() and deposit(), withdraw() reveals a multiline comment, one or more lines of documentation starting with /* and ending with */. The compiler ignores everything from /* through */. CAUTION: You cannot place a multiline comment inside another multiline comment: /*/* Nesting multiline comments is illegal! */*/ CheckingAccount’s printBalance(), deposit(), and withdraw() methods are examples of instance methods because they associate with objects. Each method is called with a hidden argument that refers to the current object. The hidden argument allows the method to access the object’s instance fields and call other instance methods. Furthermore, instance methods can access class fields and call class methods. CHAPTER 2: Learning Language Fundamentals 60 In many situations, instance methods are all that you need. However, when writing an application, you must introduce at least one class method, a method that associates with a class instead of with that class’s objects, into one of the application’s classes. Specifically, an application must specify public static void main(String[] args) to serve as the application’s entry point. The static reserved word is what makes this method a class method. (I will explain reserved word public later in this chapter.) NOTE: main() is called with an array of String objects, where each object contains the character representation of a command-line argument. Because class methods are not called with a hidden argument that refers to the current object, main() cannot access an object’s instance fields or call its instance methods. This method can only access class fields and call class methods. Many methods require you to pass a fixed number of arguments when they are called. However, Java also provides the ability to pass a variable number of arguments. To declare a method that takes a variable number of arguments, specify three consecutive periods after the type name of the method’s rightmost parameter. For example, Listing 2–11 places these periods after the double type name. Listing 2–11. Declaring a sum() method that takes a variable number of arguments double sum(double... values) { // code that sums the passed double arguments and returns the sum } You will shortly learn how to codify this method, along with the methods that were previously introduced. Later on, you will learn how to call all of these methods. Implementing Methods Listing 2–10’s empty printBalance() method accomplishes nothing. Furthermore, its empty deposit() and withdraw() methods result in compiler errors because these methods have non-void return types and do not return values. To make these methods useful and legal, you must introduce statements into their bodies. A statement is one or more instructions that perform a task. It can be expressed as a simple statement or as a compound statement. A simple statement is a single instruction terminated by a semicolon. In addition to the empty simple statement, Java supplies local variable declaration, assignment, method- call, decision, loop (repeated execution), break and continue, and method-return simple statements. (Chapter 4 introduces additional kinds of simple statements.) A compound statement is a (possibly empty) sequence of simple and other compound statements sandwiched between open and close brace delimiters. A method body is an example. Compound statements can appear wherever simple statements appear, and are alternatively referred to as blocks. CHAPTER 2: Learning Language Fundamentals 61 Empty Statements, Local Variable Declarations, Assignments, and Method Calls An empty statement is nothing followed by a semicolon. Although this kind of statement appears to be useless, it is useful, as I will explain when I discuss loop statements. A local variable declaration introduces a variable into a method’s body or other compound statement. This local variable’s lifetime and scope range from its point of declaration to the end of the compound statement in which it is declared. Only statements appearing after the declaration can access the local variable. NOTE: A local variable is similar to a parameter in that both kinds of variables only exist during a method’s execution. However, a parameter contains a passed argument, whereas a local variable contains whatever value is necessary to help the method accomplish its task. This statement is similar to an instance field declaration in that it minimally begins with a type name, continues with an identifier that names the local variable, and ends with a semicolon. By convention, a local variable’s name begins with a lowercase letter. Furthermore, the first letter of each subsequent word in a multiword local variable name is capitalized. CAUTION: You cannot declare multiple local variables with the same name in the same scope because the compiler will report an “already defined” error message. An assignment is a close relative of the local variable declaration statement. Instead of starting with a type, this statement begins with the name of a variable that has already been declared, continues with the assignment operator (=) or a compound assignment operator (such as +=), and concludes with an expression and a semicolon. A method call executes a method, possibly passing arguments and possibly receiving a value in return. Regardless of whether or not a value is returned, a method call is a statement. If you do not assign the method’s return value to a variable, the return value is lost. However, you occasionally might be more interested in what the method accomplishes than in what the method returns. Listing 2–12’s printBalance() method demonstrates these statements except for empty. Listing 2–12. Declaring and using local variables in printBalance() void printBalance() { int magnitude = (balance < 0) ? -balance : balance; String balanceRep = (balance < 0) ? "(" : ""; balanceRep += magnitude; balanceRep += (balance < 0) ? ")" : ""; System.out.println(balanceRep); } CHAPTER 2: Learning Language Fundamentals 62 Listing 2–12 introduces several statements into printBalance() to create a suitable string-based representation of the balance field’s value and output this string. If this value is positive, it is represented and printed as is. If this value is negative, its magnitude (absolute or positive value) is represented and printed between parentheses. This listing’s first two statements declare a pair of local variables: magnitude and balanceRep. An expression initializes the magnitude variable to the balance field’s magnitude, whereas another expression initializes balanceRep to "(" or "" (the empty string), depending on whether the balance field’s value is negative or positive. Unlike fields, which are initialized to default values, and parameters, which are initialized to arguments, local variables are not implicitly initialized. Before you can read a local variable’s value, you must assign a value to the variable. Otherwise, the compiler reports an error about the local variable not having been initialized. Moving on, two assignment statements follow the local variable declaration statements. These statements append a string representation of the balance field’s magnitude followed by ")" or "" to balanceRep. Alternatively, I could have expressed the first of these statements as balanceRep = balanceRep + magnitude;. Listing 2–12 concludes by calling the System.out.println() method to output balanceRep’s character sequence to the standard output device. (Chapter 1 briefly introduces the concept of standard input/output along with System.out.println() and related methods.) Decisions Listing 2–12 used the conditional operator to determine whether to assign -balance or balance to magnitude. Although this operator is useful for initializing a variable to one of two values, it cannot be used for choosing between two statements to execute. Java supplies the if-else statement for this purpose. The if-else statement has the following syntax: if (Boolean expression) statement1 else statement2 This statement consists of reserved word if, followed by a Boolean expression in parentheses, followed by a statement to execute (if the Boolean expression evaluates to true), followed by reserved word else, followed by another statement to execute (if the Boolean expression evaluates to false). Listing 2–13 demonstrates if-else. Listing 2–13. A revised printBalance() method void printBalance() { if (balance < 0) System.out.println("(" + -balance + ")"); else CHAPTER 2: Learning Language Fundamentals 63 System.out.println(balance); } Listing 2–13’s if-else statement results in the first System.out.println() method call executing if balance’s value is less than 0, and the second System.out.println() method call executing if balance’s value is greater than or equal to 0. Each of statement1 and statement2 describes another statement to execute. If you do not need the else part in the preceding syntax, you can omit else and statement2 from the syntax. The resulting statement is called if. NOTE: When if and if-else are used together, and the source code is not properly indented, it can be difficult to determine which if associates with the else. For example: if (car.door.isOpen()) if (car.key.isPresent()) car.start(); else car.door.open(); Did the developer intend for the else to match the inner if, but improperly formatted the code to make it appear otherwise? For example: if (car.door.isOpen()) if (car.key.isPresent()) car.start(); else car.door.open(); If car.door.isOpen() and car.key.isPresent() each return true, car.start() executes. If car.door.isOpen() returns true and car.key.isPresent() returns false, car.door.open(); executes. Attempting to open an open door makes no sense. The developer must have wanted the else to match the outer if, but forgot that else matches the nearest if. This problem can be fixed by surrounding the inner if with braces, as follows: if (car.door.isOpen()) { if (car.key.isPresent()) car.start(); } else car.door.open(); When car.door.isOpen() returns true, the compound statement executes. When this method returns false, car.door.open(); executes, which makes sense. CHAPTER 2: Learning Language Fundamentals 64 Forgetting that else matches the nearest if and using poor indentation to obscure this fact is known as the dangling-else problem. You can chain multiple if-else statements together, resulting in the following syntax: if (Boolean expression1) statement1 else if (Boolean expression2) statement2 else … else statementN If the first Boolean expression is true, statement1 executes. Otherwise, if the second Boolean expression is true, statement2 executes. This pattern continues until one of these expressions is true and its corresponding statement executes, or the final else is reached and statementN (the default statement) executes. Listing 2–14 demonstrates chained if-else. Listing 2–14. A revised printBalance() method using chained if-else void printBalance() { if (balance < 0) System.out.println("(" + -balance + ")"); else if (balance == 0) System.out.println("zero balance"); else System.out.println(balance); } Look closely at Listing 2–14 and you will see that its chained if-else statement is actually an if-else statement, where the statement following the else part (the first else) is another if-else statement. Chaining if-else statements together leads to verbosity that, in some cases, can be made more concise by using a switch statement. This statement lets you write code for choosing one of several statements to execute, and has the following syntax: switch (selector expression) { case value1: statement1 [break;] case value2: statement2 [break;] … case valueN: statementN [break;] [default: statement] } The switch statement consists of reserved word switch, followed by a selector expression in parentheses, followed by a body of cases. The selector expression is This book was purchased by harshamv@variable3.comCHAPTER 2: Learning Language Fundamentals 65 typically any expression that evaluates to an integral value. For example, it might evaluate to a 32–bit integer or to a 16-bit character. Each case begins with reserved word case, continues with a literal value and a colon character (:), continues with a statement to execute, and optionally concludes with a break statement (which I have yet to discuss). After evaluating the selector expression, switch compares this value with each case’s value until it finds a match. If there is a match, the case’s statement is executed. For example, if the selector expression’s value matches value1, statement1 executes. The optional break statement (anything placed in square brackets is optional), which consists of reserved word break followed by a semicolon, prevents the flow of execution from continuing with the next case’s statement. Instead, execution continues with the first statement following switch. NOTE: You will usually place a break statement after a case’s statement. Forgetting to include break can lead to a hard-to-find bug. However, there are situations where you want to group several cases together and have them execute common code. In such a situation, you would omit the break statement from the participating cases. If none of the cases’ values match the selector expression’s value, and if a default case (signified by the default reserved word followed by a colon) is present, the default case’s statement is executed. Listing 2–15 demonstrates switch. Listing 2–15. Using switch to output a compass direction class Compass { static final int NORTH = 0; static final int SOUTH = 1; static final int WEST = 2; static final int EAST = 3; void printDirection(int dir) { switch (dir) { case NORTH: System.out.println("You are travelling north."); break; case SOUTH: System.out.println("You are travelling south."); break; case EAST : System.out.println("You are travelling east."); break; case WEST : System.out.println("You are travelling west."); break; default : System.out.println("Unknown direction"); } } } Listing 2–15’s Compass class is an example of an enumerated type, a named sequence of related constants. NORTH, SOUTH, EAST, and WEST are Compass’s set of constants. CHAPTER 2: Learning Language Fundamentals 66 Java version 5 introduced the enum as an improved enumerated type that overcomes problems with the listing’s form of enumerated type. This feature includes a change to the switch statement, which I will discuss when I cover enums in Chapter 5. NOTE: Java version 7 introduces the ability to switch on a string-based selector expression. In this situation, each case’s value is a string literal. I will demonstrate this form of the switch statement in the next section. Loops It is sometimes necessary to execute a statement repeatedly. This repeated execution is called a loop. Java provides three kinds of loop statements: for, while, and do-while. The for statement has the following syntax: for ([initialize]; [test]; [update]) statement This statement consists of reserved word for, followed by a header in parentheses, followed by a statement to execute. The header consists of an optional initialization section, followed by an optional test section, followed by an optional update section. A non-optional semicolon separates each of the first two sections from the next section. The initialization section consists of a comma-separated list of local variable declarations or variable assignments. Some or all of these variables are typically used to control the loop’s duration, and are known as loop-control variables. The test section consists of a Boolean expression that determines how long the loop executes. Execution continues as long as this expression evaluates to true. Finally, the update section consists of a comma-separated list of expressions that typically modify the loop-control variables. The for statement is perfect for iterating (looping) over an array. Each iteration (loop execution) accesses one of the array’s elements via an array[index] expression, where array is the array whose element is being accessed, and index is the zero-based location of the element being accessed. Listing 2–16 uses the for statement to iterate over the array of command-line arguments that is passed to the main() method. Each argument is read from the array, and Java version 7’s enhanced switch statement uses the argument to determine a course of action. CHAPTER 2: Learning Language Fundamentals 67 Listing 2–16. Using for with switch on a string-based selector expression to process command-line arguments public static void main(String[] args) { for (int i = 0; i < args.length; i++) switch (args[i]) { case "-v": case "-V": System.out.println("version 1.0"); break; default : showUsage(); } } Listing 2–16’s for statement presents an initialization section that declares local variable i, a test section that compares i’s current value to the length of the args array (every array has a length field that returns the number of elements in the array) to ensure that this value is less than the array’s length, and an update section that increments i by 1. The loop continues until i’s value equals the array’s length. Each iteration (loop execution) accesses one of the array’s values via the args[i] expression. This expression returns the array’s ith value (which happens to be a String object in this example). The first value is stored in args[0]. The args[i] expression serves as the switch statement’s selector expression. If this String object contains -V, the second case is executed, which calls System.out.println() to output a version number message. The subsequent break statement keeps execution from falling into the default case, which calls showUsage() to output usage information when main() is called with unexpected arguments. If this String object contains -v, the lack of a break statement following the first case causes execution to fall through to the second case, calling System.out.println(). This example demonstrates the occasional need to group cases to execute common code. The while statement has the following syntax: while (Boolean expression) statement This statement consists of reserved word while, followed by a parenthesized Boolean expression header, followed by a statement to repeatedly execute. The while statement first evaluates the Boolean expression. If it is true, while executes the other statement. Once again, the Boolean expression is evaluated. If it is still true, while re-executes the statement. This cyclic pattern continues. Prompting the user to enter a specific character is one situation where while is useful. For example, suppose that you want to prompt the user to enter a specific uppercase letter or its lowercase equivalent. Listing 2–17 provides a demonstration. Listing 2–17. Prompting the user to enter a specific character via a while statement int ch = 0; while (ch != 'C' && ch != 'c') { System.out.println("Press C or c to continue."); CHAPTER 2: Learning Language Fundamentals 68 ch = System.in.read(); } Listing 2–17 begins by initializing local variable ch. This variable must be initialized; otherwise, the compiler will report an uninitialized variable when it tries to read ch’s value in the while statement’s Boolean expression. This expression uses the conditional AND operator (&&) to test ch’s value. This operator first evaluates its left operand, which happens to be expression ch != 'C'. (The != operator converts 'C' from 16-bit unsigned char type to 32–bit signed int type, prior to the comparison.) If ch does not contain C (which it does not at this point—0 was just assigned to ch), this expression evaluates to true. The && operator next evaluates its right operand, which happens to be expression ch != 'c'. Because this expression also evaluates to true, conditional AND returns true and while executes the compound statement. The compound statement first outputs, via the System.out.println() method call, a message that prompts the user to press either the C key or the c key. It next reads the entered character via System.in.read() (discussed in Chapter 1), saving the character’s integer value in ch. Following this assignment, the compound statement ends and while reevaluates its Boolean expression. Suppose ch contains C’s integer value. Conditional AND evaluates ch != 'C', which evaluates to false. Seeing that the expression is already false, conditional AND short circuits its evaluation by not evaluating its right operand, and returns false. The while statement subsequently detects this value and terminates. Suppose ch contains c’s integer value. Conditional AND evaluates ch != 'C', which evaluates to true. Seeing that the expression is true, conditional AND evaluates ch != 'c', which evaluates to false. Once again, the while statement terminates. NOTE: A for statement can be coded as a while statement. For example, for (int i = 0; i < 10; i++) System.out.println(i); is equivalent to int i = 0; while (i < 10) { System.out.println(i); i++; } CHAPTER 2: Learning Language Fundamentals 69 The do-while statement has the following syntax: do statement while(Boolean expression); This statement consists of the do reserved word, followed by a statement to repeatedly execute, followed by the while reserved word, followed by a parenthesized Boolean expression header, followed by a semicolon. The do-while statement first executes the other statement. It then evaluates the Boolean expression. If it is true, do-while executes the other statement. Once again, the Boolean expression is evaluated. If it is still true, do-while re-executes the statement. This cyclic pattern continues. Listing 2–18 demonstrates do-while in another example of prompting the user to enter a specific uppercase letter or its lowercase equivalent. Listing 2–18. Prompting the user to enter a specific character via a do-while statement int ch; do { System.out.println("Press C or c to continue."); ch = System.in.read(); } while (ch != 'C' && ch != 'c'); Listing 2–18 is similar to Listing 2–17. This time, however, the compound statement is executed prior to the test. As a result, it is no longer necessary to initialize ch—ch is assigned System.in.read()’s return value prior to the Boolean expression’s evaluation. It is sometimes useful for a loop statement to execute the empty statement repeatedly. The actual work performed by the loop statement takes place in the statement header. Listing 2–19 presents an example. Listing 2–19. Reading and outputting lines of text for (String line; (line = readLine()) != null; System.out.println(line)); Listing 2–19 uses for to present a programming idiom for copying lines of text that are read from some source, via the fictitious readLine() method in this example, to some destination, via System.out.println() in this example. Copying continues until readLine() returns null. Note the semicolon (empty statement) at the end of the line. CAUTION: Be careful with the empty statement because it can introduce subtle bugs into your code. For example, the following code fragment is supposed to output Hello on ten lines. Instead, only one instance of this string appears—the empty statement is executed ten times: for (int i = 0; i < 10; i++); // this ; represents the empty statement System.out.println("Hello"); CHAPTER 2: Learning Language Fundamentals 70 Break and Continue What do for(;;);, while(true);, and do;while(true); have in common? Each of these loop statements presents an extreme example of an infinite loop (a loop that never ends). An infinite loop is something that you should avoid because its unending execution causes your application to hang, which is not desirable from the point of view of your application’s users. CAUTION: An infinite loop can also arise from a loop header’s Boolean expression comparing a floating-point value against a nonzero value via the equality or inequality operator, because many floating-point values have inexact internal representations. For example, the following code fragment never ends because 0.1 does not have an exact internal representation: for (double d = 0.0; d != 1.0; d += 0.1) System.out.println(d); However, there are times when it is handy to code a loop as if it were infinite by using one of the aforementioned programming idioms. For example, you might code a while(true) loop that repeatedly prompts for a specific keystroke until the correct key is pressed. When the correct key is pressed, the loop must end. Java provides the break statement for this purpose. The break statement transfers execution to the first statement following a switch statement (as discussed earlier) or a loop. In either scenario, this statement consists of reserved word break followed by a semicolon. Listing 2–20 uses break with an if decision statement to exit a while(true)-based infinite loop when the user presses the C or c key. Listing 2–20. Breaking out of an infinite loop int ch; while (true) { System.out.println("Press C or c to continue."); ch = System.in.read(); if (ch == 'C' || ch == 'c') break; } The break statement is also useful in the context of a finite loop. For example, consider a scenario where an array of values is searched for a specific value, and you want to exit the loop when this value is found. Listing 2–21 reveals this scenario. CHAPTER 2: Learning Language Fundamentals 71 Listing 2–21. Prematurely breaking out of a for-based loop int[] employeeIDs = { 123, 854, 567, 912, 224 }; int employeeSearchID = 912; boolean found = false; for (int i = 0; i < employeeIDs.length; i++) if (employeeSearchID == employeeIDs[i]) { found = true; break; } System.out.println((found) ? "employee " + employeeSearchID + " exists" : "no employee ID matches " + employeeSearchID); Listing 2–21 uses for and if to search an array of employee IDs to determine if a specific employee ID exists. If this ID is found, if’s compound statement assigns true to found. Because there is no point in continuing the search, it then uses break to quit the loop. The continue statement skips the remainder of the current loop iteration, reevaluates the header’s Boolean expression, and performs another iteration (if true) or terminates the loop (if false). Continue consists of reserved word continue followed by a semicolon. Consider a while loop that reads lines from a source and processes nonblank lines in some manner. Because it should not process blank lines, while skips the current iteration when a blank line is detected, as demonstrated in Listing 2–22. Listing 2–22. Skipping the remainder of the current iteration String line; while ((line = readLine()) != null) { if (isBlank(line)) continue; processLine(line); } Listing 2–22 employs a fictitious isBlank() method to determine if the currently read line is blank. If this method returns true, if executes the continue statement to skip the rest of the current iteration and read the next line whenever a blank line is detected. Look carefully at Listing 2–22 and you should realize that the continue statement is not needed. Instead, this listing can be shortened via refactoring (rewriting source code to improve its readability, organization, or reusability), as demonstrated in Listing 2–23. Listing 2–23. A refactored if statement String line; while ((line = readLine()) != null) { if (!isBlank(line)) processLine(line); } Listing 2–23’s refactoring modifies if’s Boolean expression to use the logical complement operator (!). Whenever isBlank() returns false, this operator flips this value to true and if executes processLine(). CHAPTER 2: Learning Language Fundamentals 72 Unlike break, continue does not appear to be a necessary part of the language. As you have just seen, it is possible to remove continue by inverting the Boolean expression. If you find yourself relying too much on continue, perhaps you need to refactor your code. Java provides labeled versions of break and continue as disciplined versions of goto, a statement that transfers execution to a labeled statement. (Many developers hate goto because undisciplined use of this statement results in unreadable/unmaintainable code.) NOTE: Java reserves the goto identifier so that it cannot be used to name any source code entity. However, GOTO is not reserved, so it seems pointless to reserve goto. The labeled break statement consists of break, followed by an identifier for which a matching label (an identifier followed by a colon) must exist. Furthermore, the label must immediately precede a loop statement. Labeled break is useful for breaking out of nested loops (loops within loops). For example, Listing 2–24 reveals the labeled break statement transferring execution to the first statement that follows the outer for loop. Listing 2–24. Breaking out of nested for loops outer: for (int i = 0; i < 3; i++) for (int j = 0; j < 3; j++) if (i == 1 && j == 1) break outer; else System.out.println("i=" + i + ", j=" + j); System.out.println("Both loops terminated."); When i’s value is 1 and j’s value is 1, break outer; is executed to terminate both for loops. This statement transfers execution to the first statement after the outer for loop, which happens to be System.out.println("Both loops terminated.");. The following output is generated: i=0, j=0 i=0, j=1 i=0, j=2 i=1, j=0 Both loops terminated. The labeled continue statement consists of continue, followed by an identifier for which a matching label (an identifier followed by a colon) must exist. Furthermore, the label must immediately precede a loop statement. Labeled continue is useful for terminating the current and future iterations of nested loops and beginning a new iteration of the labeled loop. For example, Listing 2–25 reveals the labeled continue statement terminating the inner for loop’s iterations. CHAPTER 2: Learning Language Fundamentals 73 Listing 2–25. Continuing the outer for loop outer: for (int i = 0; i < 3; i++) for (int j = 0; j < 3; j++) if (i == 1 && j == 1) continue outer; else System.out.println("i=" + i + ", j=" + j); System.out.println("Both loops terminated."); When i’s value is 1 and j’s value is 1, continue outer; is executed to terminate the inner for loop and continue with the outer for loop at its next value of i. Both loops continue until they finish. The following output is generated: i=0, j=0 i=0, j=1 i=0, j=2 i=1, j=0 i=2, j=0 i=2, j=1 i=2, j=2 Both loops terminated. Method Return A void method’s execution flows from its first statement to its last statement. However, Java’s return statement lets a method exit prior to the last statement. As Listing 2–26 reveals, this statement consists of reserved word return followed by a semicolon. Listing 2–26. Returning from a method class Employee { int salary; void setSalary(int empSalary) { if (empSalary < 0) { System.out.println("salary cannot be negative"); return; } salary = empSalary; } } Listing 2–26’s setSalary() method uses an if statement to detect an attempt to assign a negative value to the salary field. In this case, an error message is output and return prematurely exits the method so that the negative value cannot be assigned. This form of the return statement is not legal in a method that returns a value. For such methods, Java provides a version of return that lets the method return a value (whose type must match the method’s return type). Listing 2–27 demonstrates this version. 3CHAPTER 2: Learning Language Fundamentals 74 Listing 2–27. Returning a value from a method int deposit(int amount) { if (amount <= 0) { System.out.println("cannot deposit a negative or zero amount"); return balance; } balance += amount; return balance; } Listing 2–27’s deposit() method uses an if statement to detect an attempt to assign a negative or zero value to the balance field (of the previously presented CheckingAccount class), and outputs an error message when this attempt is detected. Furthermore, it returns balance’s current value. If there is no problem, balance is updated and its new value is returned. I previously declared a sum() method whose parameter list indicates that this method takes a variable number of arguments. I did not present a body for this method because I had not covered statements (especially return). Now that you have been introduced to the return statement, take a look at Listing 2–28. Listing 2–28. Declaring the body of a variable-argument sum() method double sum(double... values) { int total = 0; for (int i = 0; i < values.length; i++) total += values[i]; return total; } Listing 2–28’s implementation totals the number of arguments passed to this method. (Behind the scenes, these arguments are stored in a one-dimensional array, as evidenced by values.length and values[i]). After these values have been totaled, this total is returned via the return statement. Overloaded Methods Java lets you introduce methods with the same name but different parameter lists into the same class. This feature is known as method overloading. When the compiler encounters a method call, it compares the called method’s arguments list with each overloaded method’s parameter list as it looks for the correct method to call. Two same-named methods are overloaded if their parameter lists differ in number or order of parameters. For example, Java’s String class provides overloaded public int indexOf(int ch) and public int indexOf(int ch, int fromIndex) methods. These methods differ in parameter counts. (I explore String in Chapter 7.) Two same-named methods are overloaded if at least one parameter differs in type. For example, Java’s Math class provides overloaded public static double abs(double a) This book was purchased by harshamv@variable3.comCHAPTER 2: Learning Language Fundamentals 75 and public static int abs(int a) methods. One method’s parameter is a double; the other method’s parameter is an int. (I explore Math in Chapter 6.) You cannot overload a method by changing only the return type. For example, double sum(double... values) and int sum(double... values) are not overloaded. These methods are not overloaded because the compiler does not have enough information to choose which method to call when it encounters sum(1.0, 2.0) in source code. Introducing Constructors Constructors are named blocks of code, declared in class bodies, for constructing objects by initializing their instance fields and performing other initialization tasks. A constructor declaration specifies the same name as the class and a (potentially empty) parameter list. However, it does not specify a return type. A return type is not necessary because the result of a constructor call is always a newly created object whose type is the constructor’s class. Listing 2–29 shows you how to declare a constructor. Listing 2–29. Introducing a constructor into the CheckingAccount class class CheckingAccount { String owner; int balance; static int counter; CheckingAccount(String acctOwner, int acctBalance) { owner = acctOwner; balance = acctBalance; counter++; // keep track of created CheckingAccount objects } } Listing 2–29 declares a two-parameter constructor. When this constructor is called during object creation, acctOwner and acctBalance arguments are passed to this constructor’s parameters, and subsequently assigned to CheckingAccount’s owner and balance instance fields. CAUTION: You cannot include the reserved word static in a constructor declaration because constructors are used to initialize objects—you can initialize class fields from within a constructor, but assigning an expression to a class field would probably make more sense. You cannot include the reserved word final in a constructor declaration because constructors cannot be inherited. (I discuss inheritance in the next chapter.) Constructors can be overloaded, just like regular methods. For example, Listing 2–30 reveals a second CheckingAccount constructor. CHAPTER 2: Learning Language Fundamentals 76 Listing 2–30. An overloaded CheckingAccount constructor CheckingAccount(String acctOwner) { owner = acctOwner; balance = 100; counter++; // keep track of created CheckingAccount objects } Listing 2–30’s constructor declaration specifies a single acctOwner parameter, which is assigned to the owner field. Furthermore, it assigns 100 to balance. (Perhaps this second constructor is called when creating a new checking account, where the initial balance must default to a minimum of $100.) Instead of duplicating constructor code, you can have the overloaded constructor call a previously declared constructor to reuse existing code. Listing 2–31 provides a demonstration. Listing 2–31. Calling a CheckingAccount constructor from another CheckingAccount constructor CheckingAccount(String acctOwner) { this(acctOwner, 100); } Listing 2–31’s constructor declaration also specifies a single acctOwner parameter. Furthermore, it uses reserved word this to call the previous constructor with acctOwner’s value and 100. CAUTION: You must use this to call another constructor—you cannot use the class’s name, as in CheckingAccount(). The this() constructor call (if present) must be the first statement that is executed within the constructor—this rule prevents you from specifying multiple this() constructor calls in the same constructor. Finally, you cannot specify this() in a non- constructor method—constructors can be called only by other constructors and during object creation. If a class does not declare a constructor, the class is assigned a default noargument constructor; this constructor is called during object creation. In addition to taking no arguments, this constructor’s body is empty: it contains no initialization code for initializing objects. There is no default constructor when a constructor is declared. Introducing Other Initializers You previously learned how to initialize fields by assigning expressions to them. These kinds of initializers are known as class field initializers and instance field initializers. You also learned how to use constructors to initialize instance fields. Additionally, Java supports class initializers and instance initializers. CHAPTER 2: Learning Language Fundamentals 77 Class Initializers A class initializer is a static-prefixed compound statement that is introduced into a class body. It is used to initialize a loaded class via a sequence of statements. For example, I once used a class initializer to load a custom database driver class. Listing 2– 32 shows the loading details. Listing 2–32. Loading a database driver via a class initializer class JDBCFilterDriver implements Driver { static private Driver d; static { // Attempt to load JDBC-ODBC Bridge Driver and register that // driver. try { Class c = Class.forName("sun.jdbc.odbc.JdbcOdbcDriver"); d = (Driver) c.newInstance(); DriverManager.registerDriver(new JDBCFilterDriver()); } catch (Exception e) { System.out.println(e); } } //... } Listing 2–32’s JDBCFilterDriver class uses its class initializer to load and instantiate the class that describes Java’s JDBC-ODBC Bridge Driver, and to register a JDBCFilterDriver instance with Java’s database driver. Although much of this listing is probably meaningless to you right now, it illustrates the usefulness of class initializers. Instance Initializers An instance initializer is a compound statement that is introduced into a class body, as opposed to being introduced into the body of a method or a constructor. The instance initializer is used to initialize an object via a sequence of statements, as demonstrated in Listing 2–33. Listing 2–33. Initializing a pair of arrays via an instance initializer class Graphics { double[] sines = { 0.0, 0.0, /* ... */ 0.0 }; // should be 360 entries double[] cosines = { 0.0, 0.0, /* ... */ 0.0 }; // should be 360 entries { for (int i = 0; i < sines.length; i++) { sines[i] = Math.sin(Math.toRadians(i)); cosines[i] = Math.cos(Math.toRadians(i)); } } } CHAPTER 2: Learning Language Fundamentals 78 Listing 2–33’s Graphics class uses an instance initializer to initialize an object’s sines and cosines arrays to the sines and cosines of angles ranging from 0 through (ideally) 360—not all array elements are present, for brevity. It is faster to read array elements than to repeatedly call Math.sin() and Math.cos() elsewhere; performance matters. Because the code in Listing 2–33’s instance initializer could just as easily have been placed in a constructor, what is the advantage in using an instance initializer? Apart from possibly clarifying source code, instance initializers are useful in anonymous classes (discussed Chapter 4), which cannot declare constructors. Initialization Order A class’s body can contain multiple instance field initializers, instance class initializers, constructors, instance initializers, and class initializers. Furthermore, class fields and instance fields initialize to default values. Understanding the order in which all of this initialization occurs is important to preventing confusion, so check out Listing 2–34. Listing 2–34. A complete initialization demo // InitDemo.java public class InitDemo { static boolean bool1; boolean bool2; static byte byte1; byte byte2; static char char1; char char2; static double double1; double double2; static float float1; float float2; static int int1; int int2; static long long1; long long2; static short short1; short short2; static String string1; String string2; static { System.out.println("[class] bool1 = " + bool1); System.out.println("[class] byte1 = " + byte1); System.out.println("[class] char1 = " + char1); System.out.println("[class] double1 = " + double1); System.out.println("[class] float1 = " + float1); System.out.println("[class] int1 = " + int1); System.out.println("[class] long1 = " + long1); System.out.println("[class] short1 = " + short1); System.out.println("[class] string1 = " + string1); System.out.println(); } { CHAPTER 2: Learning Language Fundamentals 79 System.out.println("[instance] bool2 = " + bool2); System.out.println("[instance] byte2 = " + byte2); System.out.println("[instance] char2 = " + char2); System.out.println("[instance] double2 = " + double2); System.out.println("[instance] float2 = " + float2); System.out.println("[instance] int2 = " + int2); System.out.println("[instance] long2 = " + long2); System.out.println("[instance] short2 = " + short2); System.out.println("[instance] string2 = " + string2); System.out.println(); } static { bool1 = true; byte1 = 127; char1 = 'A'; double1 = 1.0; float1 = 2.0F; int1 = 1000000000; long1 = 1000000000000L; short1 = 32767; string1 = "abc"; } { bool2 = true; byte2 = 127; char2 = 'A'; double2 = 1.0; float2 = 2.0F; int2 = 1000000000; long2 = 1000000000000L; short2 = 32767; string2 = "abc"; } InitDemo() { System.out.println("InitDemo() called"); System.out.println(); } static double double3 = 10.0; double double4 = 10.0; static { System.out.println("[class] double3 = " + double3); System.out.println(); } { System.out.println("[instance] double4 = " + double3); System.out.println(); } public static void main(String[] args) { System.out.println ("main() started"); System.out.println(); System.out.println("[class] bool1 = " + bool1); System.out.println("[class] byte1 = " + byte1); System.out.println("[class] char1 = " + char1); CHAPTER 2: Learning Language Fundamentals 80 System.out.println("[class] double1 = " + double1); System.out.println("[class] double3 = " + double3); System.out.println("[class] float1 = " + float1); System.out.println("[class] int1 = " + int1); System.out.println("[class] long1 = " + long1); System.out.println("[class] short1 = " + short1); System.out.println("[class] string1 = " + string1); System.out.println(); for (int i = 0; i < 2; i++) { System.out.println("About to create InitDemo object"); System.out.println(); InitDemo id = new InitDemo(); System.out.println("id created"); System.out.println(); System.out.println("[instance] id.bool2 = " + id.bool2); System.out.println("[instance] id.byte2 = " + id.byte2); System.out.println("[instance] id.char2 = " + id.char2); System.out.println("[instance] id.double2 = " + id.double2); System.out.println("[instance] id.double4 = " + id.double4); System.out.println("[instance] id.float2 = " + id.float2); System.out.println("[instance] id.int2 = " + id.int2); System.out.println("[instance] id.long2 = " + id.long2); System.out.println("[instance] id.short2 = " + id.short2); System.out.println("[instance] id.string2 = " + id.string2); System.out.println(); } } } Listing 2–34’s InitDemo class declares one class field and one instance field for each of the primitive types plus String. It also introduces one explicitly initialized class field, one explicitly initialized instance field, three class initializers, three instance initializers, and one constructor. If you compile and run this code, you will observe the following output: [class] bool1 = false [class] byte1 = 0 [class] char1 = // A char value defaults to Unicode 0, which cannot be printed. [class] double1 = 0.0 [class] float1 = 0.0 [class] int1 = 0 [class] long1 = 0 [class] short1 = 0 [class] string1 = null [class] double3 = 10.0 main() started [class] bool1 = true [class] byte1 = 127 [class] char1 = A [class] double1 = 1.0 [class] double3 = 10.0 [class] float1 = 2.0 [class] int1 = 1000000000 [class] long1 = 1000000000000 CHAPTER 2: Learning Language Fundamentals 81 [class] short1 = 32767 [class] string1 = abc About to create InitDemo object [instance] bool2 = false [instance] byte2 = 0 [instance] char2 = [instance] double2 = 0.0 [instance] float2 = 0.0 [instance] int2 = 0 [instance] long2 = 0 [instance] short2 = 0 [instance] string2 = null [instance] double4 = 10.0 InitDemo() called id created [instance] id.bool2 = true [instance] id.byte2 = 127 [instance] id.char2 = A [instance] id.double2 = 1.0 [instance] id.double4 = 10.0 [instance] id.float2 = 2.0 [instance] id.int2 = 1000000000 [instance] id.long2 = 1000000000000 [instance] id.short2 = 32767 [instance] id.string2 = abc About to create InitDemo object [instance] bool2 = false [instance] byte2 = 0 [instance] char2 = [instance] double2 = 0.0 [instance] float2 = 0.0 [instance] int2 = 0 [instance] long2 = 0 [instance] short2 = 0 [instance] string2 = null [instance] double4 = 10.0 InitDemo() called id created [instance] id.bool2 = true [instance] id.byte2 = 127 [instance] id.char2 = A [instance] id.double2 = 1.0 [instance] id.double4 = 10.0 [instance] id.float2 = 2.0 [instance] id.int2 = 1000000000 CHAPTER 2: Learning Language Fundamentals 82 [instance] id.long2 = 1000000000000 [instance] id.short2 = 32767 [instance] id.string2 = abc As you study this output, you will discover some interesting facts about initialization:  Class fields initialize to default values just after a class is loaded.  All class initialization occurs prior to the main() method being called.  Class initialization is performed in a top-down manner. (Attempting to access a class field prior to its declaration causes the compiler to report an illegal forward reference.)  Instance fields initialize to default values at the start of object creation.  All instance initialization occurs prior to a constructor returning.  Instance initialization is performed in a top-down manner. (Attempting to access an instance field prior to its declaration causes the compiler to report an illegal forward reference.) Interface Versus Implementation Every class X exposes an interface (a protocol consisting of constructors, methods, and [possibly] fields that are made available to objects created from other classes for use in creating and communicating with X’s objects). X also provides an implementation (the code within exposed methods along with optional helper methods and optional supporting fields that should not be exposed) that codifies the interface. Helper methods are methods that assist exposed methods and should not be exposed. When designing a class, your goal is to expose a useful interface while hiding details of that interface’s implementation. You hide the implementation to prevent developers from accidentally accessing parts of your class that do not belong to the class’s interface, so that you are free to change the implementation without breaking client code. Hiding the implementation is often referred to as information hiding. Furthermore, many developers consider implementation hiding to be part of encapsulation. Java supports implementation hiding by providing four levels of access control, where three of these levels are indicated via a reserved word. You can use the following access control levels to control access to fields, methods, and constructors, and two of these levels to control access to classes:  Public: A field, method, or constructor that is declared public is accessible from anywhere. Classes can be declared public as well.  Protected: A field, method, or constructor that is declared protected is accessible from all classes in the same package as the member’s class as well as subclasses of that class, regardless of package. (I will discuss packages in Chapter 4.) CHAPTER 2: Learning Language Fundamentals 83  Private: A field, method, or constructor that is declared private cannot be accessed from beyond the class in which it is declared.  Package-private: In the absence of an access control reserved word, a field, method, or constructor is only accessible to classes within the same package as the member’s class. The same is true for non-public classes. NOTE: A class that is declared public must be stored in a file with the same name. For example, a public Employee class must be stored in Employee.java. A source file can only contain one public class. You will often declare your class’s instance fields to be private and provide special public instance methods for setting and getting their values. By convention, methods that set field values have names starting with set and are known as setters. Similarly, methods that get field values have names with get (or is, for Boolean fields) prefixes and are known as getters. Listing 2–35 demonstrates this pattern in the context of an Employee class declaration. Listing 2–35. Separation of interface from implementation public class Employee { private String name; public Employee(String name) { setName(name); } public void setName(String empName) { name = empName; // Assign the empName argument to the name field. } public String getName() { return name; } } Listing 2–35 presents an interface consisting of the public constructor and public setter/getter methods. The implementation consists of the private name field and constructor/method code. It might seem pointless to go to all this bother when you could simply omit private and access the name field directly. However, suppose you are told to introduce a new constructor that takes separate first and last name arguments, and new methods that set/get the employee’s first and last names into this class. Furthermore, suppose that it has been determined that the first and last names will be accessed more often than the entire name. Listing 2–36 reveals these changes. CHAPTER 2: Learning Language Fundamentals 84 Listing 2–36. Revising implementation without affecting existing interface class Employee { private String firstName; private String lastName; Employee(String name) { setName(name); } Employee(String firstName, String lastName) { setName(firstName + " " + lastName); } void setName(String name) { // Assume that the first and last names are separated by a // single space character. indexOf() locates a character in a // string; substring() returns a portion of a string. setFirstName(name.substring(0, name.indexOf(' '))); setLastName(name.substring(name.indexOf(' ')+1)); } String getName() { return getFirstName() + " " + getLastName(); } void setFirstName(String empFirstName) { firstName = empFirstName; } String getFirstName() { return firstName; } void setLastName(String empLastName) { lastName = empLastName; } String getLastName() { return lastName; } } Listing 2–36 reveals that the name field has been removed in favor of new firstName and lastName fields, which were added to improve performance. Because setFirstName() and setLastName() will be called more frequently than setName(), and because getFirstName() and getLastName() will be called more frequently than getName(), it is faster (in each case) to have the first two methods set/get firstName’s and lastName’s values rather than having to merge either value into/extract this value from name’s value. Listing 2–36 also reveals setName() calling setFirstName() and setLastName(), and getName() calling getFirstName() and getLastName(), rather than directly accessing the firstName and lastName fields. Although avoiding direct access to these fields is not necessary in this example, imagine another implementation change that adds more This book was purchased by harshamv@variable3.comCHAPTER 2: Learning Language Fundamentals 85 code to setFirstName(), setLastName(), getFirstName(), and getLastName(); not calling these methods will result in the new code not executing. Client code (code that instantiates and uses a class, such as Employee) will not break when Employee’s implementation changes from that shown in Listing 2–35 to that shown in Listing 2–36, because the original interface remains intact, although the interface has been extended. This lack of breakage results from hiding Listing 2–35’s implementation, especially the name field. TIP: Get into the habit of developing useful interfaces while hiding implementations because it will save you much trouble when maintaining your classes. Objects You previously learned that objects are instantiated from classes. You also discovered Java’s new operator in Table 2–3, and observed brief examples of this operator’s usage in Listings 2–32 and 2–34. This section explores object and array creation via new, and also focuses on accessing fields, calling methods, and garbage collection. Creating Objects and Arrays The new operator is used to create, from a class that has already been loaded into memory, an object whose instance fields default to zero values, which you interpret as literal value false, '\u0000', 0, 0L, 0.0, 0.0F, or null (depending on field type). Furthermore, this operator calls the specified constructor to initialize instance fields to appropriate nondefault values and perform other kinds of initialization. For example, new Employee("John", "Doe") creates an object from Listing 2–36’s Employee class, and calls its Employee(String firstName, String lastName) constructor to initialize this object’s instance fields to the employee’s first and last names. The new operator creates the object in a special region of memory known as the heap. Furthermore, new returns this object’s reference (a value that helps the Java virtual machine locate the heap-based object). One virtual machine implementation might implement references as physical memory addresses, whereas another implementation might use handles (numeric identifiers that are used to locate object addresses). It does not matter how the reference is implemented because it cannot be directly accessed. About the only thing you can do with a reference is to use ==/!= to compare it with null, a special literal value that does not refer to any object. NOTE: The reference that is returned by new is represented literally by reserved word this. Wherever this appears in source code, it represents the current object. CHAPTER 2: Learning Language Fundamentals 86 References are lost unless they are stored in fields or local variables, or passed as arguments to a method. For example, Employee emp = new Employee("John", "Doe"); stores the Employee reference that new returns in Employee variable emp (the reference and variable types must agree). This variable is known as a reference variable. NOTE: The String class is treated specially by Java. For example, you have previously learned that you can assign a string literal to a String variable, as in String s = "abc";. Behind the scenes, however, Java treats this statement as a shorthand for String s = new String("abc");, in which a reference to a String object is assigned to s. (I will say more about this topic when I discuss String in Chapter 7.) The new operator is also used to create a one-dimensional array of values in the heap. When creating the array, specify new, followed by a name that identifies the type of the values that are stored in the array, followed by an integral expression between a pair of square brackets that specifies the size of the array (the number of elements). NOTE: An array is implemented as a special Java object whose read-only length field contains the array’s size. For example, you can use new to create a one-dimensional array of object references, as demonstrated in Listing 2–37. Listing 2–37. Creating a one-dimensional array of Employee object references Employee[] empArray = new Employee[10]; When you create a one-dimensional array, new zeros the bits in each array element’s storage location, which you interpret as literal value false, '\u0000', 0, 0L, 0.0, 0.0F, or null (depending on element type). In the previous example, each of empArray’s elements is initialized to null. You cannot access objects via this array until you assign nonnull object references to its elements, which Listing 2–38 demonstrates. Listing 2–38. Storing Employee object references in a one-dimensional array for (int i = 0; i < empArray.length; i++) empArray[i] = new Employee("John Doe #" + i); NOTE: You can combine new with the array initialization syntax if desired. For example, Employee[] empArray = new Employee[] {new Employee ("John Doe")}; creates a single-element Employee array initialized to a single Employee object reference. Suppose you want to use new to create a two-dimensional array, perhaps to assign to the temperatures field in Listing 2–3’s WeatherData class. The way you accomplish this task in Java is to first create a one-dimensional row array (the outer array), and then CHAPTER 2: Learning Language Fundamentals 87 create a one-dimensional column array (the inner array) for each row, as shown in Listing 2–39. Listing 2–39. Creating a two-dimensional array // Create the row array. double[][] temperatures = new double[3][]; // Note the extra empty pair of brackets. // Create a column array for each row. for (int row = 0; row < temperatures.length; row++) temperatures[row] = new double[2]; // 2 columns per row When creating the row array, you must specify an extra pair of empty brackets as part of the expression following new. (For a three-dimensional array—a one-dimensional array of tables, where this array’s elements contain row arrays—you must specify two pairs of empty brackets as part of the expression following new.) After creating a two-dimensional array, you will want to populate its elements with suitable values. For example, Listing 2–40 initializes each temperatures element, which is accessed as temperatures[row][col], to a random temperature value. Listing 2–40. Initializing a two-dimensional array for (int row = 0; row < temperatures.length; row++) for (int col = 0; col < temperatures[row].length; col++) temperatures[row][col] = Math.round(Math.random()*100); You can subsequently output these values in a tabular format by calling Listing 2–41’s for loop (this listing makes no attempt to align the temperature values in perfect columns). Listing 2–41. Outputting a two-dimensional array’s values for (int row = 0; row < temperatures.length; row++) { for (int col = 0; col < temperatures[row].length; col++) System.out.print(temperatures[row][col] + " "); System.out.println(); } Accessing Fields When you codify a class, you can often specify the field’s name as is in an expression. For example, Listing 2–12’s printBalance() instance method specifies the name of CheckingAccount’s balance instance field in statement int magnitude = (balance < 0) ? -balance : balance;. However, you cannot always access a field by simply specifying its name. For example, you cannot access a class field from another class in this manner. Also, you cannot access a field that is hidden from you. For example, you cannot access a private field from another class. The following rules will help you learn how to access fields in different contexts, and even if such access is possible: CHAPTER 2: Learning Language Fundamentals 88  Specify the name of a class field as is from anywhere within the same class as the class field. Example: counter  Specify the name of the class field’s class, followed by the member access operator (.), followed by the name of the class field from outside the class provided that its access control permits this form of access—the field is public, for example. Example: CheckingAccount.counter  Specify the name of an instance field as is from any instance method, constructor, or instance initializer in the same class as the instance field. Example: balance  Specify an object reference, followed by the member access operator, followed by the name of the instance field from any class method or class initializer within the same class as the instance field, or from outside the class provided that its access control permits this form of access. Example: CheckingAccount ca = new CheckingAccount(); int bal = ca.balance; CAUTION: Accessing an instance field (such as ca.balance) is usually not a good idea because the field exposes implementation details. However, if the class is declared as a private member of another class (a topic discussed in Chapter 4), and if its fields are frequently accessed by its enclosing class’s methods, performance is often improved if these fields are accessed directly rather than being accessed via getter method calls. Although direct access opens up implementation details to the enclosing class, this is not a problem because these details are hidden to clients of the enclosing class. Although the latter rule might seem to imply that you can access an instance field from a class context, this is not the case. Instead, you access the field from an object context. The previous access rules are not exhaustive because there exist two more access scenarios to consider: declaring a local variable (or even a parameter) with the same name as an instance field or as a class field. In either scenario, the local variable/parameter is said to shadow (hide or mask) the field. If you find that you have declared a local variable or parameter that shadows a field, you can rename the local variable/parameter, or you can use the member access operator with reserved word this (instance field) or class name (class field) to explicitly identify the field. Listing 2–42 uses this to qualify an instance field name. Listing 2–42. Qualifying the name field to circumvent shadowing by a same-named parameter class Employee { String name; void setName(String name) { this.name = name; // Assign the name argument to the name field. CHAPTER 2: Learning Language Fundamentals 89 } } Listing 2–42’s setName() method uses this.name to refer to the name field, not to setName()’s name parameter. Calling Methods When you codify a class, you can often specify the method’s name as is in a method- call statement. For example, the constructor in Listing 2–35’s Employee class specifies the name of Employee’s setName() method in method-call statement setName(name);. However, you cannot always call a method by simply specifying its name. For example, you cannot call a class method from another class in this manner. Also, you cannot call a method that is hidden from you. For example, you cannot call a private method from another class. The following rules will help you learn how to call methods in different contexts, and even if such calling is possible:  Specify the name of a class method as is from anywhere within the same class as the class method. Example: main(new String[0]); // No arguments are passed  Specify the name of the class method’s class, followed by the member access operator, followed by the name of the class method from outside the class provided that its access control permits this form of access—the method is declared public, for example. Example: Math.sin(Math.toRadians(45));  Specify the name of an instance method as is from any instance method, constructor, or instance initializer in the same class as the instance field. Example: processLine(line);  Specify an object reference, followed by the member access operator, followed by the name of the instance field from any class method or class initializer within the same class as the instance field, or from outside the class provided that its access control permits this form of access. Example: CheckingAccount ca = new CheckingAccount("John Doe"); ca.printBalance(); Although the latter rule might seem to imply that you can call an instance method from a class context, this is not the case. Instead, you call the method from an object context. NOTE: Field access and method call rules are combined in statement System.out.println();, where the leftmost member access operator accesses the out class field (of type PrintStream) in the System class, and where the rightmost member access operator calls this field’s println() method. CHAPTER 2: Learning Language Fundamentals 90 Method-Call Stack Method calls require a special area of memory known as the method-call stack. You can think of the method-call stack as a simulation of a pile of clean trays in a cafeteria—you pop (remove) the clean tray off of the top of the pile and the dishwasher will push (insert) the next clean tray onto the top of the pile. When a method is called, the virtual machine pushes its arguments and the address of the first statement to execute following the called method onto the method-call stack. The virtual machine also allocates stack space for the method’s local variables. When the method returns, the virtual machine removes local variable space, pops the address and arguments off of the stack, and transfers execution to the statement at this address. Recursive Calls A method normally executes statements that may include calls to other methods. However, it is occasionally convenient to have a method call itself. This scenario is known as recursion. For example, suppose you need to write a method that returns a factorial (the product of all the positive integers up to and including a specific integer). For example, 3! (the ! is the mathematical symbol for factorial) equals 3×2×1 or 6. Your first approach to writing this method might consist of the code presented in Listing 2–43. Listing 2–43. A nonrecursive approach to obtaining a factorial int factorial(int n) { int product = 1; for (int i = 2; i <= n; i++) product *= i; return product; } Although this code accomplishes its task, factorial() could also be written in Listing 2– 44’s recursive style. Listing 2–44. A recursive approach to obtaining a factorial int factorial(int n) { if (n == 1) return 1; // base problem else return n*factorial(n-1); } The recursive approach takes advantage of being able to express a problem in simpler terms of itself. According to Listing 2–44, the simplest problem, which is also known as the base problem, is 1! (1). CHAPTER 2: Learning Language Fundamentals 91 When an argument greater than 1 is passed to factorial(), this method breaks the problem into a simpler problem by calling itself with the next smaller argument value. Eventually, the base problem will be reached. For example, calling factorial(4) results in the following stack of expressions: 4*factorial(3) 3*factorial(2) 2*factorial(1) This last expression is at the top of the stack. When factorial(1) returns 1, these expressions are evaluated as the stack begins to unwind: 2*factorial(1) now becomes 2*1 (2) 3*factorial(2) now becomes 3*2 (6) 4*factorial(3) now becomes 4*6 (24) Recursion provides an elegant way to express many problems. Additional examples include searching tree-based data structures for specific values and, in a hierarchical file system, outputting the names of all files that contain specific text. CAUTION: Recursion consumes stack space, so make sure that your recursion eventually ends in a base problem; otherwise, you will run out of stack space and your application will terminate. Argument Passing A method call includes a list of (potentially no) arguments being passed to the method. Arguments are passed using a style of argument passing that is known as pass-by- value, which is demonstrated in Listing 2–45. Listing 2–45. A demonstration of pass-by-value Employee emp = new Employee("John Doe"); int recommendedAnnualSalaryIncrease = 1000; printReport(emp, recommendAnnualSalaryIncrease); printReport(new Employee("Jane Doe"), 1500); Pass-by-value passes the value of a variable (the reference value stored in emp or the 1000 value stored in recommendedAnnualSalaryIncrease, for example) or the value of some other expression (such as new Employee("Jane Doe") or 1500) to the method. Because of pass-by-value, you cannot assign a different Employee object’s reference to emp from inside printReport() via the printReport() parameter for this argument. After all, you have only passed a copy of emp’s value to the method. CHAPTER 2: Learning Language Fundamentals 92 Chained Instance Method Calls Two or more instance method calls can be chained together via the member access operator. For example, new CheckingAccount().deposit(1000).printBalance();. This capability lets you express code more compactly, and is also quite readable. However, you have to re-architect your methods somewhat differently, as Listing 2–46 reveals. Listing 2–46. Implementing instance methods so that calls to these methods can be chained together class CheckingAccount { int balance; CheckingAccount deposit(int amount) { balance += amount; return this; } CheckingAccount printBalance() { int magnitude = (balance < 0) ? -balance : balance; String balanceRep = (balance < 0) ? "(" : ""; balanceRep += magnitude; balanceRep += (balance < 0) ? ")" : ""; System.out.println(balanceRep); return this; } } According to Listing 2–46, you must specify the name of the class as the method’s return type. For example, each of deposit() and printBalance() must specify CheckingAccount as the return type. Also, you must specify return this; (return a reference to the current object) as the instance method’s last statement. Garbage Collection Objects are created via reserved word new, but how are they destroyed? Without some way to destroy objects, they will eventually fill up the heap’s available space and the application will not be able to continue. Java solves this problem by using a garbage collector, code that runs in the background and checks for unreferenced objects. When it discovers an unreferenced object, the garbage collector removes it from the heap, making more heap space available. An unreferenced object is an object that cannot be accessed from anywhere within an application. For example, new Employee("John", "Doe"); is an unreferenced object because the Employee reference returned by new is thrown away. In contrast, a referenced object is an object where the application stores at least one reference. For example, Employee emp = new Employee("John", "Doe"); is a referenced object because variable emp contains a reference to the Employee object. CHAPTER 2: Learning Language Fundamentals 93 A referenced object becomes unreferenced when the application removes its last stored reference. For example, if emp is a local variable that contains the only reference to an Employee object, this object becomes unreferenced when emp’s method terminates. An application can remove a stored reference by assigning null to its reference variable. For example, emp = null; removes the reference to the Employee object that was previously stored in emp. Java’s garbage collector eliminates a form of memory leakage in C++ implementations that do not rely on a garbage collector. In these implementations, the developer must destroy dynamically created objects before they go out of scope. If they vanish before destruction, they remain in the heap. Eventually, the heap fills and the application halts. Although this form of memory leakage is not a problem in Java, a related form of leakage is problematic: continually creating objects and forgetting to remove even one reference to each object causes the heap to fill up and the application to eventually come to a halt. This form of memory leakage is a major problem for applications that run for lengthy periods of time—a web server is one example. For shorter-lived applications, you will normally not notice this form of memory leakage. However, it is a good habit to assign null to reference variables when their referenced objects are no longer required. EXERCISES The following exercises are designed to test your understanding of Java’s language fundamentals: 1. What does a class declaration contain? 2. Is transient a reserved word? Is delegate a reserved word? 3. What is a variable? 4. Identify Java’s only unsigned primitive type. 5. What is the difference between an instance field and a class field? 6. What is an array? 7. How do you declare a one-dimensional array variable? How do you declare a two- dimensional array variable? 8. Define scope. 9. Is string literal "The quick brown fox \jumps\ over the lazy dog." legal or illegal? Why? 10. What is the purpose of the cast operator? 11. Which operator is used to create an object? 12. Can you nest multiline comments? CHAPTER 2: Learning Language Fundamentals 94 13. True or false: When declaring a method that takes a variable number of arguments, you must specify the three consecutive periods just after the rightmost parameter’s type name. 14. Given a two-dimensional array x, what does x.length return? 15. What is the difference between the while and do-while statements? 16. Initialize the sines and cosines array declarations using the new syntax. 17. Why is it okay for an expression assigned to an instance field to access a class field that is declared after the instance field? In contrast, it is not okay for the expression to access another instance field that is declared after the instance field. 18. What is required to create an array of objects? 19. How do you prevent a field from being shadowed? 20. How do you chain together instance method calls? 21. The factorial() method provides an example of tail recursion, a special case of recursion in which the method’s last statement contains a recursive call, which is known as the tail call. Provide another example of tail recursion. 22. Merge the various CheckingAccount code fragments into a complete application. Summary Before you can write applications, you need to understand the fundamentals of the Java language. These fundamentals revolve around the concepts of class and object, where a class is nothing more than a template from which objects are created. Classes are declared with reserved word class and a name. The class declaration’s brace-delimited body is populated with a combination of field, method, and constructor declarations. A field is a variable that stores a value. It initializes to a default value, although you can explicitly initialize it to a value with the help of an expression, which is a combination of variables, method calls, literals, and operators. There are two kinds of fields: instance and class. Instance fields are used to describe entity attributes, whereas class fields are used to describe class attributes. A class field is differentiated from an instance field by being declared static. A method is a named block of code that is called with an optional list of arguments, and possibly returns a value. Arguments are passed to parameters, which are another kind of variable. Parameters only exist while the method executes. A method uses statements to perform tasks. A statement can be a simple statement, such as empty, local variable declaration, assignment, method-call, decision, loop, break and continue, and method-return. This book was purchased by harshamv@variable3.comCHAPTER 2: Learning Language Fundamentals 95 A statement can be a compound statement, which contains simple and other compound statements that are placed between braces. A compound statement is also known as a block and can appear anywhere that a simple statement can appear. A constructor is a named body of code (it is named after its class) that constructs objects by initializing their instance fields and performing other initialization tasks. Unlike methods, constructors do not have return types because they do not return anything. Constructors (and methods) can be overloaded: they have the same names but different parameter lists. Constructors use this() to call other constructors in the same class. A class that does not declare a constructor is assigned a default noargument constructor. In addition to letting you initialize fields by assigning expressions to them, and providing constructors for initializing objects, Java provides instance and class initializers to perform object initialization (when a constructor is not available) and class initialization. Certain initialization rules are followed when multiple field initializers, constructors, instance initializers, and class initializers are mixed together in a class. For example, class initialization occurs prior to instance initialization. Every class exposes an interface and also provides an implementation that codifies that interface. When designing a class, your goal is to expose a useful interface while hiding details of that interface’s implementation. Hiding details is known as information hiding. Java supports implementation hiding by providing four levels of access control: public, protected, private, and package-private. Except for package-private, each access control level is indicated via a same-named reserved word. An object encapsulates an entity’s attributes and behaviors as initialized fields and methods. The new operator creates an object from a class and calls its constructor to initialize these fields and perform other kinds of initialization. The new operator creates the object in a special region of memory known as the heap. Furthermore, new returns this object’s reference. Virtual machine implementations can implement references as physical memory addresses or as handles. The new operator also creates a one-dimensional array of values in the heap. To create a two-dimensional array, create a one-dimensional row array and then, for each row array element, create a one-dimensional column array and assign its reference to the element. There are rules for accessing fields. For example, to access a public class field from outside the class, you must specify the name of the class field’s class, followed by the member access operator, followed by the class field’s name. A local variable or parameter shadows a field when it has the same name as the field. To access the field, either rename the local variable/parameter, or use the member access operator with reserved word this (instance field) or the class name (class field). There are also rules for calling methods. For example, to call a public class method from outside the class, you must specify the name of a class method’s class, followed by the member access operator, followed by the class method’s name and argument list. CHAPTER 2: Learning Language Fundamentals 96 Method calls require a special area of memory known as the method-call stack. When a method is called, the virtual machine pushes its arguments and the address of the first statement to execute following the called method onto the method-call stack. A method normally executes statements that may include calls to other methods. However, it is occasionally convenient to have a method call itself. This scenario is known as recursion. A method call includes a list of (potentially no) arguments being passed to the method. Arguments are passed using a style of argument passing that is known as pass-by- value. Two or more instance method calls can be chained together via the member access operator. Accomplish this task by having each method specify its class name as the method’s return type, and by having the method return this. Java uses a garbage collector to destroy objects. When it discovers an unreferenced object, the garbage collector removes it from the heap, making more heap space available. Although the garbage collector eliminates the need to explicitly destroy objects, all references to objects that are no longer needed must be nullified, or else they will remain in the heap. This scenario is known as a memory leak. Now that you know how to architect classes and work with objects, you are ready to write object-based applications. However, this knowledge is not enough to write object- oriented applications. To acquire that knowledge, you need to proceed to Chapter 3. 97 97 Chapter Learning Object-Oriented Language Features An object-based language encapsulates attributes and behaviors in objects. To be known as an object-oriented language, the language must also support inheritance and polymorphism. This chapter introduces you to Java’s language features that support these twin pillars of object orientation. Furthermore, the chapter introduces you to interfaces, Java’s ultimate abstract type mechanism. Inheritance Inheritance is a hierarchical relationship between entity categories in which one category inherits attributes and behaviors from at least one other category. For example, tiger inherits from animal (tiger is a kind of animal), car inherits from vehicle (car is a kind of vehicle), and checking account inherits from bank account (checking account is a kind of bank account). Animal, vehicle, and bank account are more generic categories; and tiger, car, and checking account are more specific categories. Java supports implementation inheritance (class extension) by providing language features for declaring and initializing classes that are extensions of existing classes. After showing you how to use these features, this section introduces you to a special class that sits at the top of Java’s class hierarchy. The section then introduces you to composition, an alternative to implementation inheritance for reusing code. Lastly, I will show you how composition can overcome problems with implementation inheritance. NOTE: Java also supports another kind of inheritance called interface inheritance. Later in this chapter, while discussing Java’s interfaces language feature, I discuss interface inheritance. 3 CHAPTER 3: Learning Object-Oriented Language Features 98 Extending Classes Java provides the reserved word extends for specifying a hierarchical relationship between two classes. For example, suppose you have a Vehicle class and want to introduce a Car class as a kind of Vehicle. Listing 3–1 uses extends to cement this relationship. Listing 3–1. Relating two classes via extends class Vehicle { // member declarations } class Car extends Vehicle { // member declarations } Listing 3–1 codifies a relationship that is known as an “is-a” relationship: a car is a kind of vehicle. In this relationship, Vehicle is known as the base class, parent class, or superclass; and Car is known as the derived class, child class, or subclass. CAUTION: You cannot extend a final class. For example, if you declared Vehicle as final class Vehicle, the compiler would report an error upon encountering class Car extends Vehicle. Developers declare their classes final when they do not want these classes to be subclassed (for security or other reasons). In addition to being capable of providing its own member declarations, Car is capable of inheriting member declarations from its Vehicle superclass. As Listing 3–2 shows, inherited members become accessible to members of the Car class. Listing 3–2. Inheriting members class Vehicle { private String make; private String model; private int year; Vehicle(String make, String model, int year) { this.make = make; this.model = model; this.year = year; } String getMake() { return make; } String getModel() { return model; } int getYear() CHAPTER 3: Learning Object-Oriented Language Features 99 { return year; } } class Car extends Vehicle { private int numWheels; Car(String make, String model, int year, int numWheels) { super(make, model, year); this.numWheels = numWheels; } public static void main(String[] args) { Car car = new Car("Ford", "Fiesta", 2009, 4); System.out.println("Make = " + car.getMake()); System.out.println("Model = " + car.getModel ()); System.out.println("Year = " + car.getYear ()); // Normally, you cannot access a private field via an object // reference. However, numWheels is being accessed from // within a method (main()) that is part of the Car class. System.out.println("Number of wheels = "+car.numWheels); } } Listing 3–2’s Vehicle class declares private fields that store a vehicle’s make, model, and year; a constructor that initializes these fields to passed arguments; and getter methods that retrieve these fields’ values. The Car subclass provides a private numWheels field, a constructor that initializes a Car object’s Vehicle and Car layers, and a main() class method for test-driving this application. Car’s constructor uses reserved word super to call Vehicle’s constructor with Vehicle- oriented arguments, and then initializes Car’s numWheels instance field. The super() call is analogous to specifying this() to call another constructor in the same class. CAUTION: The super() call can only appear in a constructor. Furthermore, it must be the first code that is specified in the constructor. If super() is not specified, and if the superclass does not have a noargument constructor, the compiler will report an error because the subclass constructor must call a noargument superclass constructor when super() is not present. Car’s main() method creates a Car object, initializing this object to a specific make, model, year, and number of wheels. Four System.out.println() method calls subsequently output this information. The first three System.out.println() method calls retrieve their pieces of information by calling the Car instance’s inherited getMake(), getModel(), and getYear() methods. The final System.out.println() method call accesses the instance’s numWheels field directly. kCHAPTER 3: Learning Object-Oriented Language Features 100 NOTE: A class whose instances cannot be modified is known as an immutable class. Vehicle is an example. If Car’s main() method, which can directly read or write numWheels, was not present, Car would also be an example of an immutable class. A class cannot inherit constructors, nor can it inherit private fields and methods. Car does not inherit Vehicle’s constructor, nor does it inherit Vehicle’s private make, model, and year fields. A subclass can override (replace) an inherited method so that the subclass’s version of the method is called instead. Listing 3–3 shows you that the overriding method must specify the same name, parameter list, and return type as the method being overridden. Listing 3–3. Overriding a method class Vehicle { private String make; private String model; private int year; Vehicle(String make, String model, int year) { this.make = make; this.model = model; this.year = year; } void describe() { System.out.println(year + " " + make + " " + model); } } class Car extends Vehicle { private int numWheels; Car(String make, String model, int year, int numWheels) { super(make, model, year); } void describe() { System.out.print("This car is a "); // Print without newline – see Chapter 1. super.describe(); } public static void main(String[] args) { Car car = new Car("Ford", "Fiesta", 2009, 4); car.describe(); } } Listing 3–3’s Car class declares a describe() method that overrides Vehicle’s describe() method to output a car-oriented description. This method uses reserved word super to call Vehicle’s describe() method via super.describe();. CHAPTER 3: Learning Object-Oriented Language Features 101 NOTE: You call a superclass method from the overriding subclass method by prefixing the method’s name with reserved word super and the member access operator. If you do not do this, you end up recursively calling the subclass’s overriding method. You can also use super and the member access operator to access non-private superclass fields from subclasses that replace these fields by declaring same-named fields. If you were to compile and run Listing 3–3, you would discover that Car’s overriding describe() method executes instead of Vehicle’s overridden describe() method, and outputs This car is a 2009 Ford Fiesta. CAUTION: You cannot override a final method. For example, if Vehicle’s describe() method was declared as final void describe(), the compiler would report an error upon encountering an attempt to override this method in the Car class. Developers declare their methods final when they do not want these methods to be overridden (for security or other reasons). Also, you cannot make an overriding method less accessible than the method it overrides. For example, if Car’s describe() method was declared as private void describe(), the compiler would report an error because private access is less accessible than the default package access. However, describe() could be made more accessible by declaring it public, as in public void describe(). Suppose you happened to replace Listing 3–3’s describe() method with the method shown in Listing 3–4. Listing 3–4. Incorrectly overriding a method void describe(String owner) { System.out.print("This car, which is owned by " + owner + ", is a "); super.describe(); } The modified Car class now has two describe() methods, the explicitly declared method in Listing 3–4 and the method inherited from Vehicle. Listing 3–4 does not override Vehicle’s describe() method. Instead, it overloads this method. The Java compiler helps you detect an attempt to overload instead of override a method at compile time by letting you prefix a subclass’s method header with the @Override annotation, as shown in Listing 3–5. (I will discuss annotations in Chapter 5.) Listing 3–5. Annotating an overriding method @Override void describe() { System.out.print("This car is a "); super.describe(); } CHAPTER 3: Learning Object-Oriented Language Features 102 Specifying @Override tells the compiler that the method overrides another method. If you overload the method instead, the compiler reports an error. Without this annotation, the compiler would not report an error because method overloading is a valid feature. TIP: Get into the habit of prefixing overriding methods with the @Override annotation. This habit will help you detect overloading mistakes much sooner. Chapter 2 discussed the initialization order of classes and objects, where you learned that class members are always initialized first, and in a top-down order (the same order applies to instance members). Implementation inheritance adds a couple more details:  A superclass’s class initializers always execute before a subclass’s class initializers.  A subclass’s constructor always calls the superclass constructor to initialize an object’s superclass layer, and then initializes the subclass layer. Java lets you extend a single class, which is commonly referred to as single inheritance. However, Java does not permit you to extend multiple classes, which is known as multiple implementation inheritance, because it leads to ambiguities. For example, suppose Java supported multiple implementation inheritance, and you decided to model a tiglon (a cross between a tiger and a lioness) via the class structure shown in Listing 3–6. Listing 3–6. Modeling a tiglon class Tiger { void describe() { // Code that outputs a description of the tiger's appearance and behaviors. } } class Lioness { void describe() { // Code that outputs a description of the lioness's appearance and behaviors. } } class Tiglon extends Tiger, Lioness { // Which describe() method does Tiglon inherit? } Listing 3–6 shows an ambiguity resulting from each of Tiger and Lioness possessing a describe() method. Which of these methods does Tiglon inherit? A related ambiguity arises from same-named fields, possibly of different types. Which field is inherited? CHAPTER 3: Learning Object-Oriented Language Features 103 The Ultimate Superclass A class that does not explicitly extend another class implicitly extends Java’s Object class (located in the java.lang package—I will discuss packages in the next chapter). For example, Listing 3–1’s Vehicle class extends Object, whereas Car extends Vehicle. Object is Java’s ultimate superclass because it serves as the ancestor of every other class, but does not itself extend any other class. Object provides a common set of methods that other classes inherit. Table 3–1 describes these methods. Table 3–1. Object’s Methods Method Description Object clone() Create and return a copy of the current object. boolean equals(Object obj) Determine if the current object is equal to the object identified by obj. void finalize() Finalize the current object. Class getClass() Return the current object’s Class object. int hashCode() Return the current object’s hash code. void notify() Wake up one of the threads that are waiting on the current object’s monitor. void notifyAll() Wake up all threads that are waiting on the current object’s monitor. String toString() Return a string representation of the current object. void wait() Cause the current thread to wait on the current object’s monitor until it is woken up via notify() or notifyAll(). void wait(long timeout) Cause the current thread to wait on the current object’s monitor until it is woken up via notify() or notifyAll(), or until the specified timeout value (in milliseconds) has elapsed, whichever comes first. void wait (long timeout, int nanos) Cause the current thread to wait on the current object’s monitor until it is woken up via notify() or notifyAll(), or until the specified timeout value (in milliseconds) plus nanos value (in nanoseconds) has elapsed, whichever comes first. I will discuss getClass(), notify(), notifyAll(), and the wait() methods in Chapter 7. CHAPTER 3: Learning Object-Oriented Language Features 104 Cloning The clone() method clones (duplicates) an object without calling a constructor. It copies each primitive or reference field’s value to its counterpart in the clone, a task known as shallow copying or shallow cloning. Listing 3–7 demonstrates this behavior. Listing 3–7. Shallowly cloning an Employee object class Employee implements Cloneable { String name; int age; Employee(String name, int age) { this.name = name; this.age = age; } public static void main(String[] args) throws CloneNotSupportedException { Employee e1 = new Employee("John Doe", 46); Employee e2 = (Employee) e1.clone(); System.out.println(e1 == e2); // Output: false System.out.println(e1.name == e2.name); // Output: true } } Listing 3–7 declares an Employee class with name and age instance fields, and a constructor for initializing these fields. The main() method uses this constructor to initialize a new Employee object’s copies of these fields to John Doe and 46. NOTE: A class must implement the Cloneable interface or its instances cannot be shallowly cloned via Object’s clone() method—this method performs a runtime check to see if the class implements Cloneable. (I will discuss interfaces later in this chapter.) If a class does not implement Cloneable, clone() throws CloneNotSupportedException. (Because CloneNotSupportedException is a checked exception, it is necessary for Listing 3–7 to satisfy the compiler by appending throws CloneNotSupportedException to the main() method’s header. I will discuss exceptions in the next chapter.) String is an example of a class that does not implement Cloneable; hence, String objects cannot be shallowly cloned. After assigning the Employee object’s reference to local variable e1, main() calls the clone() method on this variable to duplicate the object, and then assigns the resulting reference to variable e2. The (Employee) cast is needed because clone() returns Object. To prove that the objects whose references were assigned to e1 and e2 are different, main() next compares these references via == and outputs the Boolean result, which happens to be false. To prove that the Employee object was shallowly cloned, main() next compares the references in both Employee objects’ name fields via == and outputs the Boolean result, which happens to be true. This book was purchased by harshamv@variable3.comCHAPTER 3: Learning Object-Oriented Language Features 105 NOTE: Object’s clone() method was originally specified as a public method, which meant that any object could be cloned from anywhere. For security reasons, this access was later changed to protected, which means that only code within the same package as the class whose clone() method is to be called, or code within a subclass of this class (regardless of package), can call clone(). Shallow cloning is not always desirable because the original object and its clone refer to the same object via their equivalent reference fields. For example, each of Listing 3–7’s two Employee objects refers to the same String object via its name field. Although not a problem for String, whose instances are immutable, changing a mutable object via the clone’s reference field results in the original (noncloned) object seeing the same change via its equivalent reference field. For example, suppose you add a reference field named hireDate to Employee. This field is of type Date with year, month, and day fields. Because Date is mutable, you can change the contents of these fields in the Date instance assigned to hireDate. Now suppose you plan to change the clone’s date, but want to preserve the original Employee object’s date. You cannot do this with shallow cloning because the change is also visible to the original Employee object. To solve this problem, you must modify the cloning operation so that it assigns a new Date reference to the Employee clone’s hireDate field. This task, which is known as deep copying or deep cloning, is demonstrated in Listing 3–8. Listing 3–8. Deeply cloning an Employee object class Date { int year, month, day; Date(int year, int month, int day) { this.year = year; this.month = month; this.day = day; } } class Employee implements Cloneable { String name; int age; Date hireDate; Employee(String name, int age, Date hireDate) { this.name = name; this.age = age; this.hireDate = hireDate; } @Override protected Object clone() throws CloneNotSupportedException { Employee emp = (Employee) super.clone(); CHAPTER 3: Learning Object-Oriented Language Features 106 if (hireDate != null) // no point cloning a null object (one that does not exist) emp.hireDate = new Date(hireDate.year, hireDate.month, hireDate.day); return emp; } public static void main(String[] args) throws CloneNotSupportedException { Employee e1 = new Employee("John Doe", 46, new Date(2000, 1, 20)); Employee e2 = (Employee) e1.clone(); System.out.println(e1 == e2); // Output: false System.out.println(e1.name == e2.name); // Output: true System.out.println(e1.hireDate == e2.hireDate); // Output: false System.out.println(e2.hireDate.year + " " + e2.hireDate.month + " " + e2.hireDate.day); // Output: 2000 1 20 } } Listing 3–8 declares Date and Employee classes. The Date class declares year, month, and day fields and a constructor. (You can declare a comma-separated list of variables on one line provided that these variables all share the same type, which is int in this case.) The Employee class overrides the clone() method to deeply clone the hireDate field. This method first calls the Object superclass’s clone() method to shallowly clone the current Employee instance’s fields, and then stores the new instance’s reference in emp. The clone() method next assigns a new Date instance to emp’s hireDate field, where this instance’s fields are initialized to the same values as those in the original Employee object’s hireDate instance. At this point, you have an Employee clone with shallowly cloned name and age fields, and a deeply cloned hireDate field. The clone() method finishes by returning this Employee clone. NOTE: If you are not calling Object’s clone() method from an overridden clone() method (because you prefer to deeply clone reference fields and do your own shallow copying of non- reference fields), it is not necessary for the class containing the overridden clone() method to implement Cloneable, but it should implement this interface for consistency. String does not override clone(), so String objects cannot be deeply cloned. Equality The == and != operators compare two primitive values (such as integers) for equality (==) or inequality (!=). These operators also compare two references to see if they refer to the same object or not. This latter comparison is known as an identity check. You cannot use == and != to determine if two objects are logically the same (or not). For example, two Car objects with the same field values are logically equivalent. However, == reports them as unequal because of their different references. CHAPTER 3: Learning Object-Oriented Language Features 107 NOTE: Because == and != perform the fastest possible comparisons, and because string comparisons need to be performed quickly (especially when sorting a huge number of strings), the String class contains special support that allows literal strings and string-valued constant expressions to be compared via == and !=. (I will discuss this support when I present String in Chapter 7.) The following statements demonstrate these comparisons: System.out.println("abc" == "abc"); // Output: true System.out.println("abc" == "a" + "bc"); // Output: true System.out.println("abc" == "Abc"); // Output: false System.out.println("abc" != "def"); // Output: true System.out.println("abc" == new String("abc")); // Output: false Recognizing the need to support logical equality in addition to reference equality, Java provides an equals() method in the Object class. Because this method defaults to comparing references, you need to override equals() to compare object contents. Before overriding equals(), make sure that this is necessary. For example, Java’s StringBuffer class does not override equals(). Perhaps this class’s designers did not think it necessary to determine if two StringBuffer objects are logically equivalent. You cannot override equals() with arbitrary code. Doing so will probably prove disastrous to your applications. Instead, you need to adhere to the contract that is specified in the Java documentation for this method, and which I present next. The equals() method implements an equivalence relation on nonnull object references:  It is reflexive: For any nonnull reference value x, x.equals(x) returns true.  It is symmetric: For any nonnull reference values x and y, x.equals(y) returns true if and only if y.equals(x) returns true.  It is transitive: For any nonnull reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) returns true.  It is consistent: For any nonnull reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals() comparisons on the objects is modified.  For any nonnull reference value x, x.equals(null) returns false. Although this contract probably looks somewhat intimidating, it is not that difficult to satisfy. For proof, take a look at the implementation of the equals() method in Listing 3– 9’s Point class. CHAPTER 3: Learning Object-Oriented Language Features 108 Listing 3–9. Logically comparing Point objects class Point { private int x, y; Point(int x, int y) { this.x = x; this.y = y; } int getX() { return x; } int getY() { return y; } @Override public boolean equals(Object o) { if (!(o instanceof Point)) return false; Point p = (Point) o; return p.x == x && p.y == y; } public static void main(String[] args) { Point p1 = new Point(10, 20); Point p2 = new Point(20, 30); Point p3 = new Point(10, 20); // Test reflexivity System.out.println(p1.equals(p1)); // Output: true // Test symmetry System.out.println(p1.equals(p2)); // Output: false System.out.println(p2.equals(p1)); // Output: false // Test transitivity System.out.println(p2.equals(p3)); // Output: false System.out.println(p1.equals(p3)); // Output: true // Test nullability System.out.println(p1.equals(null)); // Output: false // Extra test to further prove the instanceof operator's usefulness. System.out.println(p1.equals("abc")); // Output: false } } Listing 3–9’s overriding equals() method begins with an if statement that uses the instanceof operator to determine if the argument passed to parameter o is an instance of the Point class. If not, the if statement executes return false;. The o instanceof Point expression satisfies the last portion of the contract: For any nonnull reference value x, x.equals(null) returns false. Because null is not an instance of any class, passing this value to equals() causes the expression to evaluate to false. The o instanceof Point expression also prevents a ClassCastException instance from being thrown via expression (Point) o in the event that you pass an object other than a Point object to equals(). (I will discuss exceptions in the next chapter.) CHAPTER 3: Learning Object-Oriented Language Features 109 Following the cast, the contract’s reflexivity, symmetry, and transitivity requirements are met by only allowing Points to be compared with other Points, via expression p.x == x && p.y == y. The final contract requirement, consistency, is met by making sure that the equals() method is deterministic. In other words, this method does not rely on any field value that could change from method call to method call. TIP: You can optimize the performance of a time-consuming equals() method by first using == to determine if o’s reference identifies the current object. Simply specify if (o == this) return true; as the equals() method’s first statement. This optimization is not necessary in Listing 3–9’s equals() method, which has satisfactory performance. It is important to always override the hashCode() method when overriding equals(). I did not do so in Listing 3–9 because I have yet to formally introduce hashCode(). Finalization Finalization refers to cleanup. The finalize() method’s Java documentation states that finalize() is “called by the garbage collector on an object when garbage collection determines that there are no more references to the object. A subclass overrides the finalize() method to dispose of system resources or to perform other cleanup.” Object’s version of finalize() does nothing; you must override this method with any needed cleanup code. Because the virtual machine might never call finalize() before an application terminates, you should provide an explicit cleanup method, and have finalize() call this method as a safety net in case the method is not otherwise called. CAUTION: Never depend on finalize() for releasing limited resources such as graphics contexts or file descriptors. For example, if an application object opens files, expecting that its finalize() method will close them, the application might find itself unable to open additional files when a tardy virtual machine is slow to call finalize(). What makes this problem worse is that finalize() might be called more frequently on another virtual machine, resulting in this too-many-open-files problem not revealing itself. The developer might falsely believe that the application behaves consistently across different virtual machines. If you decide to override finalize(), your object’s subclass layer must give its superclass layer an opportunity to perform finalization. You can accomplish this task by specifying super.finalize(); as the last statement in your method, which Listing 3–10 demonstrates. CHAPTER 3: Learning Object-Oriented Language Features 110 Listing 3–10. A properly coded finalize() method for a subclass protected void finalize() throws Throwable { try { // Perform subclass cleanup. } finally { super.finalize(); } } Listing 3–10’s finalize() declaration appends throws Throwable to the method header because the cleanup code might throw an exception. If an exception is thrown, execution leaves the method and, in the absence of try-finally, super.finalize(); never executes. (I will discuss exceptions and try-finally in Chapter 4.) To guard against this possibility, the subclass’s cleanup code executes in a compound statement that follows reserved word try. If an exception is thrown, Java’s exception- handling logic executes the compound statement following the finally reserved word, and super.finalize(); executes the superclass’s finalize() method. Hash Codes The hashCode() method returns a 32-bit integer that identifies the current object’s hash code, a small value that results from applying a mathematical function to a potentially large amount of data. The calculation of this value is known as hashing. You must override hashCode() when overriding equals(), and in accordance with the following contract, which is specified in hashCode()’s Java documentation:  Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode() method must consistently return the same integer, provided no information used in equals(Object) comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.  If two objects are equal according to the equals(Object) method, then calling the hashCode() method on each of the two objects must produce the same integer result.  It is not required that if two objects are unequal according to the equals(Object) method, then calling the hashCode() method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects might improve the performance of hash tables. Fail to obey this contract and your class’s instances will not work properly with Java’s hash-based collections, such as HashMap. (I will discuss collections in Chapter 8.) CHAPTER 3: Learning Object-Oriented Language Features 111 If you override equals() but not hashCode(), you most importantly violate the second item in the contract: The hash codes of equal objects must also be equal. This violation can lead to serious consequences, as demonstrated in Listing 3–11. Listing 3–11. The problem of not overriding hashCode() java.util. Map map = new java.util.HashMap(); map.put(p1, "first point"); System.out.println(map.get(p1)); // Output: first point System.out.println(map.get(new Point(10, 20))); // Output: null Assume that Listing 3–11’s statements are appended to Listing 3–9’s main() method. After main() creates its Point objects and calls its System.out.println() methods, it executes Listing 3–11’s statements, which perform the following tasks:  The first statement instantiates the HashMap class, which is located in the java.util package. (I will discuss packages in the next chapter.)  The second statement calls HashMap’s put() method to store Listing 3– 9’s p1 object key and the "first point" value in the hashmap.  The third statement retrieves the value of the hashmap entry whose Point key is logically equal to p1 via HashMap’s get() method.  The fourth statement is equivalent to the third statement, but returns the null reference instead of "first point". Although objects p1 and Point(10, 20) are logically equivalent, these objects have different hash codes, resulting in each object referring to a different entry in the hashmap. If an object is not stored (via put()) in that entry, get() returns null. Correcting this problem requires that hashCode() be overridden in order to return the same integer value for logically equivalent objects. I will show you how to accomplish this task when I discuss HashMap in Chapter 8. String Representation The toString() method returns a string-based representation of the current object. This representation defaults to the object’s class name, followed by the @ symbol, followed by a hexadecimal representation of the object’s hash code. For example, if you were to execute System.out.println(p1); to output Listing 3–9’s p1 object, you would see a line of output similar to Point@3e25a5. (System.out.println() calls p1’s inherited toString() method behind the scenes.) You should strive to override toString() so that it returns a concise but meaningful description of the object. For example, you might declare, in Listing 3–9’s Point class, a toString() method that is similar to Listing 3–12’s toString() method. Listing 3–12. Returning a meaningful string-based representation of a Point object public String toString() { return "(" + x + ", " + y + ")"; } CHAPTER 3: Learning Object-Oriented Language Features 112 This time, executing System.out.println(p1); results in more meaningful output, such as (10, 20). Composition Implementation inheritance and composition offer two different approaches to reusing code. As you have learned, implementation inheritance is concerned with extending a class with a new class, which is based upon an “is-a” relationship between them: a Car is a Vehicle, for example. On the other hand, composition is concerned with composing classes out of other classes, which is based upon a “has-a” relationship between them. For example, a Car has an Engine, Wheels, and a SteeringWheel. You have already seen examples of composition in Chapter 2 and this chapter. For example, Chapter 2’s CheckingAccount class included a String owner field. Listing 3– 13’s Car class provides another example of composition. Listing 3–13. A Car class whose instances are composed of other objects class Car extends Vehicle { private Engine engine; private Wheel[] wheels; private SteeringWheel steeringWheel; } Listing 3–13 demonstrates that composition and implementation inheritance are not mutually exclusive. Although not shown, Car inherits various members from its Vehicle superclass, in addition to providing its own engine, wheels, and steeringwheel fields. The Trouble with Implementation Inheritance Implementation inheritance is potentially dangerous, especially when the developer does not have complete control over the superclass, or when the superclass is not designed and documented with extension in mind. The problem is that implementation inheritance breaks encapsulation. The subclass relies on implementation details in the superclass. If these details change in a new version of the superclass, the subclass might break, even if the subclass is not touched. For example, suppose you have purchased a library of Java classes, and one of these classes describes an appointment calendar. Although you do not have access to this class’s source code, assume that Listing 3–14 describes part of its code. Listing 3–14. An appointment calendar class public class ApptCalendar { private final static int MAX_APPT = 1000; private Appt[] appts; private int size; CHAPTER 3: Learning Object-Oriented Language Features 113 public ApptCalendar() { appts = new Appt[MAX_APPT]; size = 0; // redundant because field automatically initialized to 0 // adds clarity, however } public void addAppt(Appt appt) { if (size == appts.length) return; // array is full appts[size++] = appt; } public void addAppts(Appt[] appts) { for (int i = 0; i < appts.length; i++) addAppt(appts[i]); } } Listing 3–14’s ApptCalendar class stores an array of appointments, with each appointment described by an Appt instance. For this discussion, the details of Appt are irrelevant. Suppose you want to log each appointment in a file. Because a logging capability is not provided, you extend ApptCalendar with Listing 3–15’s LoggingApptCalendar class, which adds logging behavior in overriding addAppt() and addAppts() methods. Listing 3–15. Extending the appointment calendar class public class LoggingApptCalendar extends ApptCalendar { // A constructor is not necessary because the Java compiler will add a // noargument constructor that calls the superclass's noargument // constructor by default. @Override public void addAppt(Appt appt) { Logger.log(appt.toString()); super.addAppt(appt); } @Override public void addAppts(Appt[] appts) { for (int i = 0; i < appts.length; i++) Logger.log(appts[i].toString()); super.addAppts(appts); } } Listing 3–15’s LoggingApptCalendar class relies on a Logger class whose log() class method logs a string to a file (the details are unimportant). Notice the use of toString() to convert an Appt object to a String object, which is then passed to log(). Although this class looks okay, it does not work as you might expect. Suppose you instantiate this class and add a few Appt instances to this instance via addAppts(), as demonstrated in Listing 3–16. CHAPTER 3: Learning Object-Oriented Language Features 114 Listing 3–16. Demonstrating the logging appointment calendar LoggingApptCalendar lapptc = new LoggingApptCalendar(); lapptc.addAppts(new Appt[] {new Appt(), new Appt(), new Appt()}); If you also add a System.out.println() method call to Logger’s log() method, to output this method’s argument, you will discover that log() outputs a total of six messages; each of the expected three messages (one per Appt object) is duplicated. When LoggingApptCalendar’s addAppts() method is called, it first calls Logger.log() for each Appt instance in the appts array that is passed to addAppts(). This method then calls ApptCalendar’s addAppts() method via super.addAppt(appt);. ApptCalendar’s addAppts() method calls LoggingApptCalendar’s overriding addAppt() method for each Appt instance in its appts array argument. addAppt() calls Logger.log() to log its appt argument, and you end up with three additional logged messages. If you did not override the addAppts() method, this problem would go away. However, the subclass would be tied to an implementation detail: ApptCalendar’s addAppts() method calls addAppt(). It is not a good idea to rely on an implementation detail when the detail is not documented. (I previously stated that you do not have access to ApptCalendar’s source code.) When a detail is not documented, it can change in a new version of the class. Because a base class change can break a subclass, this problem is known as the fragile base class problem. A related cause of fragility that also has to do with overriding methods occurs when new methods are added to a superclass in a subsequent release. For example, suppose a new version of the library introduces a new public void addAppt(Appt appt, boolean unique) method into the ApptCalendar class. This method adds the appt instance to the calendar when unique is false, and, when unique is true, adds the appt instance only if it has not previously been added. Because this method has been added after the LoggingApptCalendar class was created, LoggingApptCalendar does not override the new addAppt() method with a call to Logger.log(). As a result, Appt instances passed to the new addAppt() method are not logged. Here is another problem: You introduce a method into the subclass that is not also in the superclass. A new version of the superclass presents a new method that matches the subclass method signature and return type. Your subclass method now overrides the superclass method, and probably does not fulfill the superclass method’s contract. There is a way to make these problems disappear. Instead of extending the superclass, create a private field in a new class, and have this field reference an instance of the superclass. This task demonstrates composition because you are forming a has-a relationship between the new class and the superclass. Additionally, have each of the new class’s instance methods call the corresponding superclass method via the superclass instance that was saved in the private field, and also return the called method’s return value. This task is known as forwarding, and the new methods are known as forwarding methods. This book was purchased by harshamv@variable3.comCHAPTER 3: Learning Object-Oriented Language Features 115 Listing 3–17 presents an improved LoggingApptCalendar class that uses composition and forwarding to forever eliminate the fragile base class problem and the additional problem of unanticipated method overriding. Listing 3–17. A composed logging appointment calendar class public class LoggingApptCalendar { private ApptCalendar apptCal; public LoggingApptCalendar(ApptCalendar apptCal) { this.apptCal = apptCal; } public void addAppt(Appt appt) { Logger.log(appt.toString()); apptCal.addAppt(appt); } public void addAppts(Appt[] appts) { for (int i = 0; i < appts.length; i++) Logger.log(appts[i].toString()); apptCal.addAppts(appts); } } Listing 3–17’s LoggingApptCalendar class does not depend upon implementation details of the ApptCalendar class. You can add new methods to ApptCalendar and they will not break LoggingApptCalendar. NOTE: LoggingApptCalendar is an example of a wrapper class, a class whose instances wrap other instances. Each LoggingApptCalendar instance wraps an ApptCalendar instance. LoggingApptCalendar is also an example of the Decorator design pattern, which is presented on page 175 of Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (Addison-Wesley, 1995; ISBN: 0201633612). When should you extend a class and when should you use a wrapper class? Extend a class when an is-a relationship exists between the superclass and the subclass, and either you have control over the superclass or the superclass has been designed and documented for class extension. Otherwise, use a wrapper class. What does “design and document for class extension” mean? Design means provide protected methods that hook into the class’s inner workings (to support writing efficient subclasses), and ensure that constructors and the clone() method never call overridable methods. Document means clearly state the impact of overriding methods. CHAPTER 3: Learning Object-Oriented Language Features 116 CAUTION: Wrapper classes should not be used in a callback framework, an object framework in which an object passes its own reference to another object (via this) so that the latter object can call the former object’s methods at a later time. This “calling back to the former object’s method” is known as a callback. Because the wrapped object does not know of its wrapper class, it passes only its reference (via this), and resulting callbacks do not involve the wrapper class’s methods. Polymorphism Polymorphism is the ability to change forms. Examples of polymorphism abound in nature. For example, water is naturally a liquid, but it changes to a solid when frozen, and it changes to a gas when heated to its boiling point. Java supports several kinds of polymorphism:  Coercion: An operation serves multiple types through implicit type conversion. For example, division lets you divide an integer by another integer, or divide a floating-point value by another floating-point value. If one operand is an integer and the other operand is a floating-point value, the compiler coerces (implicitly converts) the integer to a floating-point value, to prevent a type error. (There is no division operation that supports an integer operand and a floating-point operand.) Passing a subclass object reference to a method’s superclass parameter is another example of coercion polymorphism. The compiler coerces the subclass type to the superclass type, to restrict operations to those of the superclass.  Overloading: The same operator symbol or method name can be used in different contexts. For example, + can be used to perform integer division, floating-point division, or string concatenation, depending on the types of its operands. Also, multiple methods having the same name can appear in a class (through declaration and/or inheritance).  Parametric: Within a class declaration, a field name can associate with different types and a method name can associate with different parameter and return types. The field and method can then take on different types in each class instance. For example, a field might be of type Integer and a method might return an Integer in one class instance, and the same field might be of type String and the same method might return a String in another class instance. Java supports parametric polymorphism via generics, which I will discuss in Chapter 5. CHAPTER 3: Learning Object-Oriented Language Features 117  Subtype: A type can serve as another type’s subtype. When a subtype instance appears in a supertype context, executing a supertype operation on the subtype instance results in the subtype’s version of that operation executing. For example, suppose that Circle is a subclass of Point, and that both classes contain a draw() method. Assigning a Circle instance to a variable of type Point, and then calling the draw() method via this variable, results in Circle’s draw() method being called. Many developers do not regard coercion and overloading as valid kinds of polymorphism. They see coercion and overloading as nothing more than type conversions and syntactic sugar (syntax that simplifies a language, making it “sweeter” to use). In contrast, parametric and subtype are regarded as valid polymorphisms. This section focuses on subtype polymorphism by first examining upcasting and late binding. The section then introduces you to abstract classes and abstract methods, downcasting and runtime type identification, and covariant return types. Upcasting and Late Binding Listing 3–9’s Point class represents a point as an x-y pair. Because a circle (in this example) is an x-y pair denoting its center, and has a radius denoting its extent, you can extend Point with a Circle class that introduces a radius field. Check out Listing 3–18. Listing 3–18. A Circle class extending the Point class class Circle extends Point { private int radius; Circle(int x, int y, int radius) { super(x, y); this.radius = radius; } int getRadius() { return radius; } } The fact that Circle is really a Point with a radius implies that you can treat a Circle instance as if it was a Point instance. Accomplish this task by assigning the Circle instance to a Point variable, as demonstrated in Listing 3–19. Listing 3–19. Upcasting from Circle to Point Circle c = new Circle(10, 20, 30); Point p = c; The cast operator is not needed to convert from Circle to Point because access to a Circle instance via Point’s interface is legal. After all, a Circle is at least a Point. This assignment is known as upcasting because you are implicitly casting up the type hierarchy (from the Circle subclass to the Point superclass). CHAPTER 3: Learning Object-Oriented Language Features 118 After upcasting Circle to Point, you cannot call Circle’s getRadius() method because this method is not part of Point’s interface. Losing access to subtype features after narrowing it to a superclass seems useless, but is necessary for achieving subtype polymorphism. In addition to upcasting the subclass instance to a variable of the superclass type, subtype polymorphism involves declaring a method in the superclass and overriding this method in the subclass. For example, suppose Point and Circle are to be part of a graphics application, and you need to introduce a draw() method into each class to draw a point and a circle, respectively. You end with the class structure shown in Listing 3–20. Listing 3–20. Declaring a graphics application’s Point and Circle classes class Point { private int x, y; Point(int x, int y) { this.x = x; this.y = y; } int getX() { return x; } int getY() { return y; } @Override public String toString() { return "(" + x + ", " + y + ")"; } void draw() { System.out.println("Point drawn at " + toString ()); } } class Circle extends Point { private int radius; Circle(int x, int y, int radius) { super(x, y); this.radius = radius; } int getRadius() { return radius; } @Override public String toString() { return "" + radius; } CHAPTER 3: Learning Object-Oriented Language Features 119 @Override void draw() { System.out.println("Circle drawn at " + super.toString() + " with radius " + toString()); } } Although the draw() methods will ultimately draw graphics shapes, simulating their behaviors via System.out.println() method calls is sufficient during the early testing phase of the graphics application. Now that you have temporarily finished with Point and Circle, you want to test their draw() methods in a simulated version of the graphics application. To achieve this objective, you write Listing 3–21’s Graphics class. Listing 3–21. A Graphics class for testing Point’s and Circle’s draw() methods class Graphics { public static void main(String[] args) { Point[] points = new Point[] {new Point(10, 20), new Circle(10, 20, 30)}; for (int i = 0; i < points.length; i++) points[i].draw(); } } Listing 3–21’s main() method first declares an array of Points. Upcasting is demonstrated by first having the array’s initializer instantiate the Circle class, and then by assigning this instance’s reference to the second element in the points array. Moving on, main() uses a for loop to call each Point element’s draw() method. Because the first iteration calls Point’s draw() method, whereas the second iteration calls Circle’s draw() method, you observe the following output: Point drawn at (10, 20) Circle drawn at (10, 20) with radius 30 How does Java “know” that it must call Circle’s draw() method on the second loop iteration? Should it not call Point’s draw() method because Circle is being treated as a Point thanks to the upcast? At compile time, the compiler does not know which method to call. All it can do is verify that a method exists in the superclass, and verify that the method call’s arguments list and return type match the superclass’s method declaration. In lieu of knowing which method to call, the compiler inserts an instruction into the compiled code that, at runtime, fetches and uses whatever reference is in points[1] to call the correct draw() method. This task is known as late binding. Late binding is used for calls to non-final instance methods. For all other method calls, the compiler knows which method to call, and inserts an instruction into the compiled code that calls the method associated with the variable’s type (not its value). This task is known as early binding. CHAPTER 3: Learning Object-Oriented Language Features 120 Abstract Classes and Abstract Methods Suppose new requirements dictate that your graphics application must include a Rectangle class. Furthermore, this class must include a draw() method, and this method must be tested in a manner similar to that shown in Listing 3–21’s Graphics class. In contrast to Circle, which is a Point with a radius, it does not make sense to think of a Rectangle as a being a Point with a width and height. Rather, a Rectangle instance would probably be composed of a Point (indicating its origin) and a width and height. Because circles, points, and rectangles are examples of shapes, it makes more sense to declare a Shape class with its own draw() method than to specify class Rectangle extends Point. Listing 3–22 presents Shape’s declaration. Listing 3–22. Declaring a Shape class class Shape { void draw() {} } You can now refactor Point to extend Listing 3–22’s Shape class, leave Circle as is, and introduce a Rectangle class that extends Shape. You can then refactor Listing 3–21’s Graphics class’s main() method to take Shape into account. Check out Listing 3–23. Listing 3–23. A new main() method for the Graphics class takes Shape into account public static void main(String[] args) { Shape[] shapes = new Shape[] {new Point(10, 20), new Circle(10, 20, 30), new Rectangle(20, 30, 15, 25)}; for (int i = 0; i < shapes.length; i++) shapes[i].draw(); } Because Point and Rectangle directly extend Shape, and because Circle indirectly extends Shape by extending Point, Listing 3–23’s main() method will call the appropriate subclass’s draw() method in response to shapes[i].draw();. Although the introduction of Shape makes our code more flexible, there is a problem. What is to stop us from instantiating Shape and adding this meaningless instance to the shapes array, as Listing 3–24 demonstrates? Listing 3–24. A useless instantiation Shape[] shapes = new Shape[] {new Point(10, 20), new Circle(10, 20, 30), new Rectangle(20, 30, 15, 25), new Shape()}; What does it mean to instantiate Shape? Because this class describes an abstract concept, what does it mean to draw a generic shape? Fortunately, Java provides a solution to this problem, which is demonstrated in Listing 3–25. Listing 3–25. Abstracting the Shape class abstract class Shape { abstract void draw(); // semicolon is required } CHAPTER 3: Learning Object-Oriented Language Features 121 Listing 3–25 uses Java’s abstract reserved word to declare a class that cannot be instantiated. The compiler reports an error should you try to instantiate this class. TIP: Get into the habit of declaring classes that describe generic categories (such as shape, animal, vehicle, and account) abstract. This way, you will not inadvertently instantiate them. The abstract reserved word is also used to declare a method without a body. The draw() method does not need a body because it cannot draw an abstract shape. CAUTION: The compiler reports an error if you attempt to declare a class that is both abstract and final. For example, abstract final class Shape is an error because an abstract class cannot be instantiated and a final class cannot be extended. The compiler also reports an error if you declare a method to be abstract but do not declare its class to be abstract. For example, removing abstract from the Shape class’s header in Listing 3–25 results in an error. This removal is an error because a non-abstract (concrete) class cannot be instantiated if it contains an abstract method. When you extend an abstract class, the extending class must override all of the abstract class’s abstract methods, or else the extending class must itself be declared to be abstract; otherwise, the compiler will report an error. An abstract class can contain non-abstract methods in addition to or instead of abstract methods. For example, Listing 3–2’s Vehicle class could have been declared abstract. The constructor would still be present, to initialize private fields, even though you could not instantiate the resulting class. Downcasting and Runtime Type Identification Moving up the type hierarchy via upcasting results in loss of access to subtype features. For example, assigning a Circle instance to Point variable p means that you cannot use p to call Circle’s getRadius() method. However, it is possible to once again access the Circle instance’s getRadius() method by performing an explicit cast operation; for example, Circle c = (Circle) p;. This assignment is known as downcasting because you are explicitly moving down the type hierarchy (from the Point superclass to the Circle subclass). Although an upcast is always safe (the superclass’s interface is a subset of the subclass’s interface), the same cannot be said of a downcast. Listing 3–26 shows you what kind of trouble you can get into when downcasting is used incorrectly. CHAPTER 3: Learning Object-Oriented Language Features 122 Listing 3–26. The trouble with downcasting class A { } class B extends A { void d() {} } class C { public static void main(String[] args) { A a = new A(); B b = (B) a; b.d(); } } Listing 3–26 presents a class hierarchy consisting of a superclass named A and a subclass named B. Although A does not declare any members, B declares a single d() method. A third class named C provides a main() method that first instantiates A, and then tries to downcast this instance to B and assign the result to variable b. The compiler will not complain because downcasting from a superclass to a subclass in the same type hierarchy is legal. However, if the assignment is allowed, the application will undoubtedly crash when it tries to execute b.d();. The crash happens because the virtual machine will attempt to call a method that does not exist—class A does not have a d() method. Fortunately, this scenario will never happen because the virtual machine verifies that the cast is legal. Because it detects that A does not have a d() method, it does not permit the cast by throwing an instance of the ClassCastException class. The virtual machine’s cast verification illustrates runtime type identification (or RTTI, for short). Cast verification performs RTTI by examining the type of the cast operator’s operand to see if the cast should be allowed. Clearly, the cast should not be allowed. A second form of RTTI involves the instanceof operator. This operator checks the left operand to see if it is an instance of the right operand, and returns true if this is the case. Listing 3–27 introduces instanceof to Listing 3–26 to prevent the ClassCastException. Listing 3–27. Preventing a ClassCastException if(a instanceof B) { B b = (B) a; b.d(); } The instanceof operator detects that variable a’s instance was not created from B and returns false to indicate this fact. As a result, the code that performs the illegal cast will not execute. (Overuse of instanceof probably indicates poor software design.) CHAPTER 3: Learning Object-Oriented Language Features 123 Because a subtype is a kind of supertype, instanceof will return true when its left operand is a subtype instance or a supertype instance of its right operand supertype. Listing 3–28 provides a demonstration. Listing 3–28. Subtype and supertype instances of a supertype A a = new A(); B b = new B(); System.out.println(b instanceof A); // Output: true System.out.println(a instanceof A); // Output: true Listing 3–28, which assumes the class structure shown in Listing 3–26, instantiates superclass A and subclass B. The first System.out.println() method call outputs true because b’s reference identifies an instance of a subclass of A; the second System.out.println() method call outputs true because a’s reference identifies an instance of superclass A. So far, you have encountered two forms of RTTI. Java also supports a third form that is known as reflection. I will introduce you to this form of RTTI when I cover reflection in Chapter 7. Covariant Return Types A covariant return type is a method return type that, in the superclass’s method declaration, is the supertype of the return type in the subclass’s overriding method declaration. Listing 3–29 provides a demonstration of this language feature. Listing 3–29. A demonstration of covariant return types class Zip { ZipFile getArchive(String name) throws IOException { return new ZipFile(name); // ZipFile is located in the java.util.zip package } } class Jar extends Zip { @Override JarFile getArchive(String name) throws IOException { return new JarFile(name); // JarFile is located in the java.util.jar package } } class Archive { public static void main(String[] args) throws IOException { if (args.length == 2 && args[0].equals("-zip")) { ZipFile zf = new Zip().getArchive(args[1]); } else if (args.length == 2 && args[0].equals("-jar")) { JarFile jf = new Jar().getArchive(args[1]); CHAPTER 3: Learning Object-Oriented Language Features 124 } } } Listing 3–29 declares a Zip superclass and a Jar subclass; each class declares a getArchive() method. Zip’s method has its return type set to ZipFile, whereas Jar’s overriding method has its return type set to JarFile, a subclass of ZipFile. Covariant return types minimize upcasting and downcasting. For example, Jar’s getArchive() method does not need to upcast its JarFile instance to its JarFile return type. Furthermore, this instance does not need to be downcast to JarFile when assigning to variable jf. In the absence of covariant return types, you would end up with Listing 3–30. Listing 3–30. Upcasting and downcasting in the absence of covariant return types class Zip { ZipFile getArchive(String name) throws IOException { return new ZipFile(name); } } class Jar extends Zip { @Override ZipFile getArchive(String name) throws IOException { return new JarFile(name); } } class Archive2 { public static void main(String[] args) throws IOException { if (args.length == 2 && args[0].equals("-zip")) { ZipFile zf = new Zip().getArchive(args[1]); } else if (args.length == 2 && args[0].equals("-jar")) { JarFile jf = (JarFile) new Jar().getArchive(args[1]); } } } In Listing 3–30, the first bolded code reveals an upcast from JarFile to ZipFile, and the second bolded code uses the required (JarFile) cast operator to downcast from ZipFile to jf, which is of type JarFile. This book was purchased by harshamv@variable3.comCHAPTER 3: Learning Object-Oriented Language Features 125 Interfaces In Chapter 2, I stated that every class X exposes an interface, which is a protocol or contract consisting of constructors, methods, and (possibly) fields that are made available to objects created from other classes for use in creating and communicating with X’s objects. NOTE: A contract is an agreement between two parties. In this case, those parties are a class and clients (external constructors, methods, class initializers, and instance initializers) that communicate with the class’s instances by calling constructors and methods, and by accessing fields (typically public static final fields, or constants). The essence of the contract is that the class promises to not change its interface, which would break clients that depend upon the interface. Java formalizes the interface concept by providing reserved word interface, which is used to introduce a type without implementation. Java also provides language features to declare, implement, and extend interfaces. After looking at interface declaration, implementation, and extension, this section explains the rationale for using interfaces. Declaring Interfaces An interface declaration consists of a header followed by a body. At minimum, the header consists of reserved word interface followed by a name that identifies the interface. The body starts with an open brace character and ends with a close brace. Sandwiched between these delimiters are constant and method header declarations. Consider Listing 3–31. Listing 3–31. Declaring a Drawable interface interface Drawable { int RED = 1; // For simplicity, integer constants are used. These constants are int GREEN = 2; // not that descriptive, as you will see. int BLUE = 3; int BLACK = 4; void draw(int color); } Listing 3–31 declares an interface named Drawable. By convention, an interface’s name begins with an uppercase letter. Furthermore, the first letter of each subsequent word in a multiword interface name is capitalized. CHAPTER 3: Learning Object-Oriented Language Features 126 NOTE: Many interface names end with the able suffix. For example, the Java’s standard class library includes interfaces named Adjustable, Callable, Comparable, Cloneable, Iterable, Runnable, and Serializable. It is not mandatory to use this suffix. For example, the standard class library also provides interfaces named CharSequence, Collection, Composite, Executor, Future, Iterator, List, Map, and Set. Drawable declares four fields that identify color constants. Drawable also declares a draw() method that must be called with one of these constants to specify the color used to draw something. NOTE: As with a class declaration, you can precede interface with public, to make your interface accessible to code outside of its package. (I will discuss packages in the next chapter). Otherwise, the interface is only accessible to other types in its package. You can also precede interface with abstract, to emphasize that an interface is abstract. Because an interface is already abstract, it is redundant to specify abstract in the interface’s declaration. An interface’s fields are implicitly declared public, static, and final. It is therefore redundant to declare them with these reserved words. Because these fields are constants, they must be explicitly initialized; otherwise, the compiler reports an error. An interface’s methods are implicitly declared public and abstract. Therefore, it is redundant to declare them with these reserved words. Because these methods must be instance methods, do not declare them static or the compiler will report errors. Drawable identifies a type that specifies what to do (draw something) but not how to do it. Implementation details are left up to classes that implement this interface. Instances of such classes are known as drawables because they know how to draw themselves. NOTE: An interface that declares no members is known as a marker interface or a tagging interface. It associates metadata with a class. For example, the Cloneable marker/tagging interface states that instances of its implementing class can be shallowly cloned. RTTI is used to detect that an object’s class implements a marker/tagging interface. For example, when Object’s clone() method detects, via RTTI, that the calling instance’s class implements Cloneable, it shallowly clones the object. CHAPTER 3: Learning Object-Oriented Language Features 127 Implementing Interfaces By itself, an interface is useless. To be of any benefit to an application, the interface needs to be implemented by a class. Java provides the implements reserved word for this task. This reserved word is demonstrated in Listing 3–32. Listing 3–32. Implementing the Drawable interface class Point implements Drawable { private int x, y; Point(int x, int y) { this.x = x; this.y = y; } int getX() { return x; } int getY() { return y; } @Override public String toString() { return "(" + x + ", " + y + ")"; } @Override public void draw(int color) { System.out.println("Point drawn at " + toString () + " in color " + color); } } class Circle extends Point implements Drawable { private int radius; Circle(int x, int y, int radius) { super(x, y); this.radius = radius; } int getRadius() { return radius; } @Override public String toString() { return "" + radius; } @Override public void draw(int color) { System.out.println("Circle drawn at " + super.toString() + " with radius " + toString() + " in color " + color); } } CHAPTER 3: Learning Object-Oriented Language Features 128 Listing 3–32 retrofits Listing 3–20’s class hierarchy to take advantage of Listing 3–31’s Drawable interface. You will notice that each of classes Point and Circle implements this interface by attaching the implements Drawable clause to its class header. To implement an interface, the class must specify, for each interface method header, a method whose header has the same signature and return type as the interface’s method header, and a code body to go with the method header. CAUTION: When implementing a method, do not forget that the interface’s methods are implicitly declared public. If you forget to include public in the implemented method’s declaration, the compiler will report an error because you are attempting to assign weaker access to the implemented method. When a class implements an interface, the class inherits the interface’s constants and method headers, and overrides the method headers by providing implementations (hence the @Override annotation). This is known as interface inheritance. It turns out that Circle’s header does not need the implements Drawable clause. If this clause is not present, Circle inherits Point’s draw() method, and is still considered to be a Drawable, whether it overrides this method or not. An interface specifies a type whose data values are the objects whose classes implement the interface, and whose behaviors are those specified by the interface. This fact implies that you can assign an object’s reference to a variable of the interface type, provided that the object’s class implements the interface. Listing 3–33 provides a demonstration. Listing 3–33. Exercising the Drawable interface public static void main(String[] args) { Drawable[] drawables = new Drawable[] {new Point(10, 20), new Circle(10, 20, 30)}; for (int i = 0; i < drawables.length; i++) drawables[i].draw(Drawable.RED); } Because Point and Circle instances are drawables by virtue of these classes implementing the Drawable interface, it is legal to assign Point and Circle instance references to variables (including array elements) of type Drawable. When you run this method, it generates the following output: Point drawn at (10, 20) in color 1 Circle drawn at (10, 20) with radius 30 in color 1 Listing 3–31’s Drawable interface is useful for drawing a shape’s outline. Suppose you also need to fill a shape’s interior. You might attempt to satisfy this requirement by declaring Listing 3–34’s Fillable interface. CHAPTER 3: Learning Object-Oriented Language Features 129 Listing 3–34. Declaring a Fillable interface interface Fillable { int RED = 1; int GREEN = 2; int BLUE = 3; int BLACK = 4; void fill(int color); } You can declare that the Point and Circle classes implement both interfaces by specifying class Point implements Drawable, Fillable and class Circle implements Drawable, Fillable. TIP: You can list as many interfaces as you need to implement by specifying a comma-separated list of interface names after implements. Implementing multiple interfaces can lead to name collisions, and the compiler will report errors. For example, suppose that you attempt to compile Listing 3–35’s interface and class declarations. Listing 3–35. Colliding interfaces interface A { int X = 1; void foo(); } interface B { int X = 1; int foo(); } class C implements A, B { public void foo(); public int foo() { return X; } } Each of interfaces A and B declares a constant named X. Despite each constant having the same type and value, the compiler will report an error when it encounters X in C’s second foo() method because it does not know which X is being inherited. Speaking of foo(), the compiler reports an error when it encounters C’s second foo() declaration because foo() has already been declared. You cannot overload a method by changing only its return type. The compiler will probably report additional errors. For example, the Java version 6 update 16 compiler has this to say when told to compile Listing 3–35: X.java:14: foo() is already defined in C public int foo() { return X; } ^ CHAPTER 3: Learning Object-Oriented Language Features 130 X.java:11: C is not abstract and does not override abstract method foo() in B class C implements A, B ^ X.java:13: foo() in C cannot implement foo() in B; attempting to use incompatible return type found : void required: int public void foo(); ^ X.java:14: reference to X is ambiguous, both variable X in A and variable X in B match public int foo() { return X; } ^ 4 errors Extending Interfaces Just as a subclass can extend a superclass via reserved word extends, you can use this reserved word to have a subinterface extend a superinterface. This is known as interface inheritance. For example, the duplicate color constants in Drawable and Fillable lead to name collisions when you specify their names by themselves in an implementing class. To avoid these name collisions, prefix a name with its interface name and the member access operator, or place these constants in their own interface, and have Drawable and Fillable extend this interface, as demonstrated in Listing 3–36. Listing 3–36. Extending the Colors interface interface Colors { int RED = 1; int GREEN = 2; int BLUE = 3; int BLACK = 4; } interface Drawable extends Colors { void draw(int color); } interface Fillable extends Colors { void fill(int color); } The fact that Drawable and Fillable each inherit constants from Colors is not a problem for the compiler. There is only a single copy of these constants (in Colors) and no possibility of a name collision, and so the compiler is satisfied. If a class can implement multiple interfaces by declaring a comma-separated list of interface names after implements, it seems that an interface should be able to extend multiple interfaces in a similar way. This feature is demonstrated in Listing 3–37. CHAPTER 3: Learning Object-Oriented Language Features 131 Listing 3–37. Extending a pair of interfaces interface A { int X = 1; } interface B { double X = 2.0; } interface C extends A, B { } Listing 3–37 will compile even though C inherits two same-named constants X with different return types and initializers. However, if you implement C and then try to access X, as in Listing 3–38, you will run into a name collision. Listing 3–38. Discovering a name collision class D implements C { public void output() { System.out.println(X); // Which X is accessed? } } Suppose you introduce a void foo(); method header declaration into interface A, and an int foo(); method header declaration into interface B. This time, the compiler will report an error when you attempt to compile the modified Listing 3–37. Why Use Interfaces? Now that the mechanics of declaring, implementing, and extending interfaces are out of the way, we can focus on the rationale for using them. Unfortunately, newcomers to Java’s interfaces feature are often told that this feature was created as a workaround to Java’s lack of support for multiple implementation inheritance. While interfaces are useful in this capacity, this is not their reason for existence. Instead, Java’s interfaces feature was created to give developers the utmost flexibility in designing their applications, by decoupling interface from implementation. If you are an adherent to agile software development (a group of software development methodologies based on iterative development that emphasizes keeping code simple, testing frequently, and delivering functional pieces of the application as soon as they are deliverable), you know the importance of flexible coding. You know that you cannot afford to tie your code to a specific implementation because a change in requirements for the next iteration could result in a new implementation, and you might find yourself rewriting significant amounts of code, which wastes time and slows development. Interfaces help you achieve flexibility by decoupling interface from implementation. For example, Listing 3–23’s main() method creates an array of objects from classes that CHAPTER 3: Learning Object-Oriented Language Features 132 subclass the Shape class, and then iterates over these objects, calling each object’s draw() method. The only objects that can be drawn are those that subclass Shape. Suppose you also have a hierarchy of classes that model resistors, transistors, and other electronic components. Each component has its own symbol that allows the component to be shown in a schematic diagram of an electronic circuit. Perhaps you want to add a drawing capability to each class that draws that component’s symbol. You might consider specifying Shape as the superclass of the electronic component class hierarchy. However, electronic components are not shapes so it makes no sense to place these classes in a class hierarchy rooted in Shape. However, you can make each component class implement the Drawable interface, which lets you add expressions that instantiate these classes to Listing 3–33’s drawables array (so you can draw their symbols). This is legal because these instances are drawables. Wherever possible, you should strive to specify interfaces instead of classes in your code, to keep your code adaptable to change. This is especially true when working with Java’s collections framework, which I will discuss at length in Chapter 8. For now, consider a simple example that consists of the collections framework’s List interface, and its ArrayList and LinkedList classes. Listing 3–39 shows you an example of inflexible code based on the ArrayList class. Listing 3–39. Hardwiring the ArrayList class into source code ArrayList arrayList = new ArrayList(); void dump(ArrayList arrayList) { // suitable code to dump out the arrayList } Listing 3–39 uses the generics-based parameterized type language feature (which I will discuss in Chapter 5) to identify the kind of objects stored in an ArrayList instance. In this example, String objects are stored. Listing 3–39 is inflexible because it hardwires the ArrayList class into multiple locations. This hardwiring focuses the developer into thinking specifically about array lists instead of generically about lists. Lack of focus is problematic when a requirements change, or perhaps a performance issue brought about by profiling (analyzing a running application to check its performance), suggests that the developer should have used LinkedList. Listing 3–39 only requires a minimal number of changes to satisfy the new requirement. In contrast, a larger code base might need many more changes. Although you only need to change ArrayList to LinkedList, to satisfy the compiler, consider changing arrayList to linkedList, to keep semantics (meaning) clear—you might have to change multiple occurrences of names that refer to an ArrayList instance throughout the source code. The developer is bound to lose time while refactoring the code to adapt to LinkedList. Instead, time could have been saved by writing Listing 3–39 to use the equivalent of CHAPTER 3: Learning Object-Oriented Language Features 133 constants. In other words, Listing 3–39 could have been written to rely on interfaces, and to only specify ArrayList in one place. Listing 3–40 shows you what the resulting code would look like. Listing 3–40. Using List to minimize referrals to the ArrayList implementation class List list = new ArrayList(); void dump(List list) { // suitable code to dump out the list } Listing 3–40 is much more flexible than Listing 3–39. If a requirements or profiling change suggests that LinkedList should be used instead of ArrayList, simply replace Array with Linked and you are done. You do not even have to change the parameter name. NOTE: Java provides interfaces and abstract classes for describing abstract types (types that cannot be instantiated). Abstract types represent abstract concepts (drawable and shape, for example), and instances of such types would be meaningless. Interfaces promote flexibility through lack of implementation—Drawable and List illustrate this flexibility. They are not tied to any single class hierarchy, but can be implemented by any class in any hierarchy. Abstract classes support implementation, but can be genuinely abstract (Listing 3–25’s abstract Shape class, for example). However, they are limited to appearing in the upper levels of class hierarchies. Interfaces and abstract classes can be used together. For example, the collections framework provides List, Map, and Set interfaces; and AbstractList, AbstractMap, and AbstractSet abstract classes that provide skeletal implementations of these interfaces. The skeletal implementations make it easy for you to create your own interface implementations, to address your unique requirements. If they do not meet your needs, you can optionally have your class directly implement the appropriate interface. EXERCISES The following exercises are designed to test your understanding of Java’s object-oriented language features: 1. What is implementation inheritance? 2. How does Java support implementation inheritance? 3. Can a subclass have two or more superclasses? CHAPTER 3: Learning Object-Oriented Language Features 134 4. How do you prevent a class from being subclassed? 5. True or false: The super() call can appear in any method. 6. If a superclass declares a constructor with one or more parameters, and if a subclass constructor does not use super() to call that constructor, why does the compiler report an error? 7. What is an immutable class? 8. True or false: A class can inherit constructors. 9. What does it mean to override a method? 10. What is required to call a superclass method from its overriding subclass method? 11. How do you prevent a method from being overridden? 12. Why can you not make an overriding subclass method less accessible than the superclass method it is overriding? 13. How do you tell the compiler that a method overrides another method? 14. Why does Java not support multiple implementation inheritance? 15. What is the name of Java’s ultimate superclass? 16. What is the purpose of the clone() method? 17. When does Object’s clone() method throw CloneNotSupportedException? 18. Explain the difference between shallow copying and deep copying. 19. Can the == operator be used to determine if two objects are logically equivalent? Why or why not? 20. What does Object’s equals() method accomplish? 21. Does expression "abc" == "a" + "bc" return true or false? 22. How can you optimize a time-consuming equals() method? 23. What is the purpose of the finalize() method? 24. Should you rely on finalize() for closing open files? Why or why not? 25. What is a hash code? 26. True or false: You should override the hashCode() method whenever your override the equals() method. 27. What does Object’s toString() method return? 28. Why should you override toString()? 29. What is composition? 30. True or false: Composition is used to implement is-a relationships and implementation inheritance is used to describe has-a relationships. 31. Identify the fundamental problem of implementation inheritance. How do you fix this problem? This book was purchased by harshamv@variable3.comCHAPTER 3: Learning Object-Oriented Language Features 135 32. What is subtype polymorphism? 33. How is subtype polymorphism accomplished? 34. Why would you use abstract classes and abstract methods? 35. Can an abstract class contain concrete methods? 36. What is the purpose of downcasting? 37. List the three forms of RTTI. 38. What is a covariant return type? 39. How do you formally declare an interface? 40. True or false: You can precede an interface declaration with the abstract reserved word. 41. What is a marker interface? 42. What is interface inheritance? 43. How do you implement an interface? 44. What problem might you encounter when you implement multiple interfaces? 45. How do you form a hierarchy of interfaces? 46. Why is Java’s interfaces feature so important? 47. What do interfaces and abstract classes accomplish? 48. How do interfaces and abstract classes differ? 49. Model part of an animal hierarchy by declaring Animal, Bird, Fish, AmericanRobin, DomesticCanary, RainbowTrout, and SockeyeSalmon classes:  Animal is public and abstract, declares private String-based kind and appearance fields, declares a public constructor that initializes these fields to passed-in arguments, declares public and abstract eat() and move() methods that take no arguments and whose return type is void, and overrides the toString() method to output the contents of kind and appearance.  Bird is public and abstract, extends Animal, declares a public constructor that passes its kind and appearance parameter values to its superclass constructor, overrides its eat() method to output eats seeds and insects (via System.out.println()), and overrides its move() method to output flies through the air.  Fish is public and abstract, extends Animal, declares a public constructor that passes its kind and appearance parameter values to its superclass constructor, overrides its eat() method to output eats krill, algae, and insects, and overrides its move() method to output swims through the water. CHAPTER 3: Learning Object-Oriented Language Features 136  AmericanRobin is public, extends Bird, and declares a public noargument constructor that passes "americanrobin" and "red breast" to its superclass constructor.  DomesticCanary is public, extends Bird, and declares a public noargument constructor that passes "domesticcanary" and "yellow, orange, black, brown, white, red" to its superclass constructor.  RainbowTrout is public, extends Fish, and declares a public noargument constructor that passes "rainbowtrout" and "bands of brilliant speckled multicolored stripes running nearly the whole length of its body" to its superclass constructor.  SockeyeSalmon is public, extends Fish, and declares a public noargument constructor that passes "sockeyesalmon" and "bright red with a green head" to its superclass constructor. For brevity, I have omitted from the Animal hierarchy abstract Robin, Canary, Trout, and Salmon classes that generalize robins, canaries, trout, and salmon. Perhaps you might want to include these classes in the hierarchy. Although this exercise illustrates the accurate modeling of a natural scenario using inheritance, it also reveals the potential for class explosion—too many classes may be introduced to model a scenario, and it might be difficult to maintain all of these classes. Keep this in mind when modeling with inheritance. 50. Continuing from the previous exercise, declare an Animals class with a main() method. This method first declares an animals array that is initialized to AmericanRobin, RainbowTrout, DomesticCanary, and SockeyeSalmon objects. The method then iterates over this array, first outputting animals[i] (which causes toString() to be called), and then calling each object’s eat() and move() methods (demonstrating subtype polymorphism). 51. Continuing from the previous exercise, declare a public Countable interface with a String getID() method. Modify Animal to implement Countable and have this method return kind’s value. Modify Animals to initialize the animals array to AmericanRobin, RainbowTrout, DomesticCanary, SockeyeSalmon, RainbowTrout, and AmericanRobin objects. Also, introduce code that computes a census of each kind of animal. This code will use the Census class that is declared in Listing 3–41. Listing 3–41. The Census class stores census data on four kinds of animals public class Census { public final static int SIZE = 4; private String[] IDs; private int[] counts; public Census() { IDs = new String[SIZE]; counts = new int[SIZE]; } public String get(int index) { return IDs[index] + " " + counts[index]; CHAPTER 3: Learning Object-Oriented Language Features 137 } public void update(String ID) { for (int i = 0; i < IDs.length; i++) { // If ID not already stored in the IDs array (which is indicated by // the first null entry that is found), store ID in this array, and // also assign 1 to the associated element in the counts array, to // initialize the census for that ID. if (IDs[i] == null) { IDs[i] = ID; counts[i] = 1; return; } // If a matching ID is found, increment the associated element in // the counts array to update the census for that ID. if (IDs[i].equals(ID)) { counts[i]++; return; } } } } Summary An understanding of Java’s fundamental language features must take inheritance and polymorphism into account. Java supports two forms of inheritance: implementation via class extension, and interface via interface implementation or interface extension. Java supports four kinds of polymorphism: coercion, overloading, parametric, and subtype. Subtype polymorphism is used to invoke subclass methods via references to subclass objects that are stored in variables of the superclass type. Java’s interfaces feature is essential for writing extremely flexible code. It achieves this flexibility by decoupling interface from implementation. Classes that implement an interface provide their own implementations. You now have enough language knowledge to write interesting Java applications, but Java’s advanced language features related to nested types, packages, static imports, and exceptions help simplify this task. Chapter 4 focuses on these feature categories. CHAPTER 3: Learning Object-Oriented Language Features 138 139 139 Chapter Mastering Advanced Language Features Part 1 Chapters 2 and 3 laid a foundation for learning the Java language. Chapter 4 builds onto this foundation by introducing you to some of Java’s more advanced language features, specifically those features related to nested types, packages, static imports, and exceptions. Additional advanced language features are covered in Chapter 5. Nested Types Classes that are declared outside of any class are known as top-level classes. Java also supports nested classes, which are classes declared as members of other classes or scopes. Nested classes help you implement top-level class architecture. There are four kinds of nested classes: static member classes, nonstatic member classes, anonymous classes, and local classes. The latter three categories are known as inner classes. This section introduces you to static member classes and inner classes. For each kind of nested class, I provide you with a brief introduction, an abstract example, and a more practical example. The section then briefly examines the topic of nesting interfaces within classes. Static Member Classes A static member class is a static member of an enclosing class. Although enclosed, it does not have an enclosing instance of that class, and cannot access the enclosing class’s instance fields and call its instance methods. However, it can access or call static members of the enclosing class, even those members that are declared private. Listing 4–1 presents a static member class declaration. 4 CHAPTER 4: Mastering Advanced Language Features Part 1 140 Listing 4–1. Declaring a static member class public class EnclosingClass { private static int i; private static void m1() { System.out.println(i); // Output: 1 } public static void m2() { EnclosedClass.accessEnclosingClass(); } public static class EnclosedClass { public static void accessEnclosingClass() { i = 1; m1(); } public void accessEnclosingClass2() { m2(); } } } Listing 4–1 declares a top-level class named EnclosingClass with class field i, class methods m1() and m2(), and static member class EnclosedClass. Also, EnclosedClass declares class method accessEnclosingClass() and instance method accessEnclosingClass2(). Because accessEnclosingClass() is declared static, m2() must prefix this method’s name with EnclosedClass and the member access operator to call this method. Also, EnclosingClass must be part of the prefix when calling this method from beyond this class. For example, EnclosingClass.EnclosedClass.accessEnclosingClass();. Because accessEnclosingClass2() is nonstatic, it must be called from an instance of EnclosedClass. For example, when calling this method from beyond EnclosingClass, you might specify EnclosingClass.EnclosedClass ec = new EnclosingClass.EnclosedClass(); ec.accessEnclosingClass2();. Static member classes have their uses. For example, Listing 4–2’s Double and Float static member classes provide different implementations of their enclosing Rectangle class. The Float version occupies less memory because of its 32-bit float fields, and the Double version provides greater accuracy because of its 64–bit double fields. Listing 4–2. Using static member classes to declare multiple implementations of their enclosing class public abstract class Rectangle { public abstract double getX(); public abstract double getY(); public abstract double getWidth(); public abstract double getHeight(); public static class Double extends Rectangle CHAPTER 4: Mastering Advanced Language Features Part 1 141 { private double x, y, width, height; public Double(double x, double y, double width, double height) { this.x = x; this.y = y; this.width = width; this.height = height; } public double getX() { return x; } public double getY() { return y; } public double getWidth() { return width; } public double getHeight() { return height; } } public static class Float extends Rectangle { private float x, y, width, height; public Float(float x, float y, float width, float height) { this.x = x; this.y = y; this.width = width; this.height = height; } public double getX() { return x; } public double getY() { return y; } public double getWidth() { return width; } public double getHeight() { return height; } } // Prevent subclassing. Use the type-specific Double and Float // implementation subclass classes to instantiate. private Rectangle() {} public boolean contains(double x, double y) { return (x >= getX() && x < getX()+getWidth()) && (y >= getY() && y < getY()+getHeight()); } } Listing 4–2’s Rectangle class demonstrates nested subclasses. Each of the Double and Float static member classes subclass the abstract Rectangle class, providing private floating-point or double precision floating-point fields, and overriding Rectangle’s abstract methods to return these fields’ values as doubles. Rectangle is abstract because it makes no sense to instantiate this class. Because it also makes no sense to directly extend Rectangle with new implementations (the Double and Float nested subclasses should be sufficient), its default constructor is declared private. Instead, you must instantiate Rectangle.Float (to save memory) or Rectangle.Double (when accuracy is required). Check out Listing 4–3. Listing 4–3. Creating and using different Rectangle implementations public static void main(String[] args) { Rectangle r = new Rectangle.Double(10.0, 10.0, 20.0, 30.0); System.out.println("x = " + r.getX()); System.out.println("y = " + r.getY()); CHAPTER 4: Mastering Advanced Language Features Part 1 142 System.out.println("width = " + r.getWidth()); System.out.println("height = " + r.getHeight()); System.out.println("contains(15.0, 15.0) = " + r.contains(15.0, 15.0)); System.out.println("contains(0.0, 0.0) = " + r.contains(0.0, 0.0)); System.out.println(); r = new Rectangle.Float(10.0f, 10.0f, 20.0f, 30.0f); System.out.println("x = " + r.getX()); System.out.println("y = " + r.getY()); System.out.println("width = " + r.getWidth()); System.out.println("height = " + r.getHeight()); System.out.println("contains(15.0, 15.0) = " + r.contains(15.0, 15.0)); System.out.println("contains(0.0, 0.0) = " + r.contains(0.0, 0.0)); } This method generates the following output: x = 10.0 y = 10.0 width = 20.0 height = 30.0 contains(15.0, 15.0) = true contains(0.0, 0.0) = false x = 10.0 y = 10.0 width = 20.0 height = 30.0 contains(15.0, 15.0) = true contains(0.0, 0.0) = false Java’s class library contains many static member classes. For example, the Character class (in the java.lang package) encloses a static member class named Subset whose instances represent subsets of the Unicode character set. AbstractMap.SimpleEntry, ObjectInputStream.GetField, and KeyStore.PrivateKeyEntry are other examples. NOTE: When you compile an enclosing class that contains a static member class, the compiler creates a classfile for the static member class whose name consists of its enclosing class’s name, a dollar-sign character, and the static member class’s name. For example, compile Listing 4–1 and you will discover EnclosingClass$EnclosedClass.class in addition to EnclosingClass.class. This format also applies to nonstatic member classes. Nonstatic Member Classes A nonstatic member class is a non-static member of an enclosing class. Each instance of the nonstatic member class implicitly associates with an instance of the enclosing class. The nonstatic member class’s instance methods can call instance methods in the enclosing class and access the enclosing class instance’s nonstatic fields. Listing 4–4 presents a nonstatic member class declaration. CHAPTER 4: Mastering Advanced Language Features Part 1 143 Listing 4–4. Declaring a nonstatic member class public class EnclosingClass { private int i; private void m1() { System.out.println(i); // Output: 1 } public class EnclosedClass { public void accessEnclosingClass() { i = 1; m1(); } } } Listing 4–4 declares a top-level class named EnclosingClass with instance field i, instance method m1(), and nonstatic member class EnclosedClass. Furthermore, EnclosedClass declares instance method accessEnclosingClass(). Because accessEnclosingClass() is nonstatic, EnclosedClass must be instantiated before this method can be called. This instantiation must take place via an instance of EnclosingClass. Listing 4–5 accomplishes these tasks. Listing 4–5. Calling a nonstatic member class’s instance method public class NSMCDemo { public static void main(String[] args) { EnclosingClass ec = new EnclosingClass(); ec.new EnclosedClass().accessEnclosingClass(); } } Listing 4–5’s main() method first instantiates EnclosingClass and saves its reference in local variable ec. Then, main() uses this reference as a prefix to the new operator, to instantiate EnclosedClass, whose reference is then used to call accessEnclosingClass(). NOTE: Prefixing new with a reference to the enclosing class is rare. Instead, you will typically call an enclosed class’s constructor from within a constructor or an instance method of its enclosing class. Suppose you need to maintain a to-do list of items, where each item consists of a name and a description. After some thought, you create Listing 4–6’s ToDo class to implement these items. Listing 4–6. Implementing to-do items as name-description pairs public class ToDo { private String name; CHAPTER 4: Mastering Advanced Language Features Part 1 144 private String desc; public ToDo(String name, String desc) { this.name = name; this.desc = desc; } public String getName() { return name; } public String getDesc() { return desc; } public String toString() { return "Name = " + getName() + ", Desc = " + getDesc(); } } You next create a ToDoList class to store ToDo instances. ToDoList uses its ToDoArray nonstatic member class to store ToDo instances in a growable array—you do not know how many instances will be stored, and Java arrays have fixed lengths. See Listing 4–7. Listing 4–7. Storing a maximum of two ToDo instances in a ToDoArray instance public class ToDoList { private ToDoArray toDoArray; private int index = 0; public ToDoList() { toDoArray = new ToDoArray(2); } public boolean hasMoreElements() { return index < toDoArray.size(); } public ToDo nextElement() { return toDoArray.get(index++); } public void add(ToDo item) { toDoArray.add(item); } private class ToDoArray { private ToDo[] toDoArray; private int index = 0; ToDoArray(int initSize) { toDoArray = new ToDo[initSize]; } void add(ToDo item) { if (index >= toDoArray.length) { This book was purchased by harshamv@variable3.comCHAPTER 4: Mastering Advanced Language Features Part 1 145 ToDo[] temp = new ToDo[toDoArray.length*2]; for (int i = 0; i < toDoArray.length; i++) temp[i] = toDoArray[i]; toDoArray = temp; } toDoArray[index++] = item; } ToDo get(int i) { return toDoArray[i]; } int size() { return index; } } } In addition to providing an add() method to store ToDo instances in the ToDoArray instance, ToDoList provides hasMoreElements() and nextElement() methods to iterate over and return the stored instances. Listing 4–8 demonstrates these methods. Listing 4–8. Creating a list of ToDo instances and iterating over this list public static void main(String[] args) { ToDoList toDoList = new ToDoList(); toDoList.add(new ToDo("#1", "Do laundry.")); toDoList.add(new ToDo("#2", "Buy groceries.")); toDoList.add(new ToDo("#3", "Vacuum apartment.")); toDoList.add(new ToDo("#4", "Write report.")); toDoList.add(new ToDo("#5", "Wash car.")); while (toDoList.hasMoreElements()) System.out.println(toDoList.nextElement()); } This method generates the following output: Name = #1, Desc = Do laundry. Name = #2, Desc = Buy groceries. Name = #3, Desc = Vacuum apartment. Name = #4, Desc = Write report. Name = #5, Desc = Wash car. Java’s class library presents many examples of nonstatic member classes. For example, the java.util package’s HashMap class declares private HashIterator, ValueIterator, KeyIterator, and EntryIterator classes for iterating over a hashmap’s values, keys, and entries. (I will discuss HashMap in Chapter 8.) NOTE: Code within an enclosed class can obtain a reference to its enclosing class instance by qualifying reserved word this with the enclosing class’s name and the member access operator. For example, if code within accessEnclosingClass() needed to obtain a reference to its EnclosingClass instance, it would specify EnclosingClass.this. CHAPTER 4: Mastering Advanced Language Features Part 1 146 Anonymous Classes An anonymous class is a class without a name. Furthermore, it is not a member of its enclosing class. Instead, an anonymous class is simultaneously declared (as an anonymous extension of a class or as an anonymous implementation of an interface) and instantiated any place where it is legal to specify an expression. Listing 4–9 demonstrates an anonymous class declaration and instantiation. Listing 4–9. Declaring and instantiating an anonymous class that extends a class abstract class Speaker { abstract void speak(); } public class ACDemo { public static void main(final String[] args) { new Speaker() { String msg = (args.length == 1) ? args[0] : "nothing to say"; void speak() { System.out.println(msg); } } .speak(); } } Listing 4–9 introduces an abstract class named Speaker and a concrete class named ACDemo. The latter class’s main() method declares an anonymous class that extends Speaker and overrides its speak() method. When this method is called, it outputs main()’s first command-line argument or a default message if there are no arguments. An anonymous class does not have a constructor (because the anonymous class does not have a name). However, its classfile does contain a hidden method that performs instance initialization. This method calls the superclass’s noargument constructor (prior to any other initialization), which is the reason for specifying Speaker() after new. Anonymous class instances should be able to access the surrounding scope’s local variables and parameters. However, an instance might outlive the method in which it was conceived (as a result of storing the instance’s reference in a field), and try to access local variables and parameters that no longer exist after the method returns. Because Java cannot allow this illegal access, which would most likely crash the virtual machine, it lets an anonymous class instance only access local variables and parameters that are declared final. Upon encountering a final local variable/parameter name in an anonymous class instance, the compiler does one of two things: CHAPTER 4: Mastering Advanced Language Features Part 1 147  If the variable’s type is primitive (int or double, for example), the compiler replaces its name with the variable’s read-only value.  If the variable’s type is reference (String, for example), the compiler introduces, into the classfile, a synthetic variable (a manufactured variable) and code that stores the local variable’s/parameter’s reference in the synthetic variable. Listing 4–10 demonstrates an alternative anonymous class declaration and instantiation. Listing 4–10. Declaring and instantiating an anonymous class that implements an interface interface Speakable { void speak(); } public class ACDemo { public static void main(final String[] args) { new Speakable() { String msg = (args.length == 1) ? args[0] : "nothing to say"; public void speak() { System.out.println(msg); } } .speak(); } } Listing 4–10 is very similar to Listing 4–9. However, instead of subclassing a Speaker class, this listing’s anonymous class implements an interface named Speakable. Apart from the hidden method calling Object() (interfaces have no constructors), Listing 4–10 behaves like Listing 4–9. Although an anonymous class does not have a constructor, you can provide an instance initializer to handle complex initialization. For example, new Office() {{addEmployee(new Employee("John Doe"));}}; instantiates an anonymous subclass of Office and adds one Employee object to this instance by calling Office’s addEmployee() method. You will often find yourself creating and instantiating anonymous classes for their convenience. For example, suppose you need to return a list of all filenames having the .java suffix. Listing 4–11 shows you how an anonymous class simplifies using the java.io package’s File and FilenameFilter classes to achieve this objective. Listing 4–11. Using an anonymous class instance to return a list of files with .java extensions String[] list = new File(directory).list(new FilenameFilter() { public boolean accept(File f, String s) { return s.endsWith(".java"); } }); CHAPTER 4: Mastering Advanced Language Features Part 1 148 NOTE: An instance of an anonymous class is similar to a closure, which is a first-class function (a method not declared in a class) with free variables that are bound in the lexical environment (surrounding scope). A first-class function is a function that can be passed as an argument to or returned from a method. A free variable is a variable referred to in a function that is not a local variable or a parameter. Think of this variable as a placeholder. Despite their similarity, there are two key differences between these language features. First, anonymous classes are more syntactically verbose than closures. Second, an anonymous class instance does not really close over its surrounding scope, because Java cannot allow the anonymous class instance to access non-final local variables and parameters. Java version 7 will introduce closures, although the exact syntax and implementation are unknown at the time of writing. However, Baptiste Wicht revealed Oracle’s first attempt at implementing closures via his May 29, 2010 blog post “Java 7: Oracle pushes a first version of closures” (http://www.baptiste-wicht.com/2010/05/oracle-pushes-a-first- version-of-closures/). Local Classes A local class is a class that is declared anywhere that a local variable is declared. Furthermore, it has the same scope as a local variable. Unlike an anonymous class, a local class has a name and can be reused. Like anonymous classes, local classes only have enclosing instances when used in nonstatic contexts. A local class instance can access the surrounding scope’s local variables and parameters. However, the local variables and parameters that are accessed must be declared final. For example, Listing 4–12’s local class declaration accesses a final parameter and a final local variable. Listing 4–12. Declaring a local class public class EnclosingClass { public void m(final int x) { final int y = x*2; class LocalClass { int a = x; int b = y; } LocalClass lc = new LocalClass(); System.out.println(lc.a); System.out.println(lc.b); } } CHAPTER 4: Mastering Advanced Language Features Part 1 149 Listing 4–12 declares EnclosingClass with its instance method m() declaring a local class named LocalClass. This local class declares a pair of instance fields (a and b) that are initialized to the values of final parameter x and final local variable y when LocalClass is instantiated: new EnclosingClass().m(10);, for example. Local classes help improve code clarity because they can be moved closer to where they are needed. For example, Listing 4–13 declares an Iterator interface and a ToDoList class whose iterator() method returns an instance of its local Iter class as an Iterator instance (because Iter implements Iterator). Listing 4–13. The Iterator interface and the ToDoList class public interface Iterator { boolean hasMoreElements(); Object nextElement(); } public class ToDoList { private ToDo[] toDoList; private int index = 0; public ToDoList(int size) { toDoList = new ToDo[size]; } public Iterator iterator() { class Iter implements Iterator { int index = 0; public boolean hasMoreElements() { return index < toDoList.length; } public Object nextElement() { return toDoList[index++]; } } return new Iter(); } public void add(ToDo item) { toDoList[index++] = item; } } Because each of Iterator and ToDoList is declared public, these types need to be stored in separate source files. Listing 4–14’s main() method demonstrates this revised ToDoList class, Listing 4–6’s ToDo class, and Iterator. CHAPTER 4: Mastering Advanced Language Features Part 1 150 Listing 4–14. Creating a list of ToDo instances and iterating over this list public static void main(String[] args) { ToDoList toDoList = new ToDoList(5); toDoList.add(new ToDo("#1", "Do laundry.")); toDoList.add(new ToDo("#2", "Buy groceries.")); toDoList.add(new ToDo("#3", "Vacuum apartment.")); toDoList.add(new ToDo("#4", "Write report.")); toDoList.add(new ToDo("#5", "Wash car.")); Iterator iter = toDoList.iterator(); while (iter.hasMoreElements()) System.out.println(iter.nextElement()); } The Iterator instance that is returned from iterator() returns ToDo items in the same order as when they were added to the list. Although you can only use the returned Iterator once, you can call iterator() whenever you need a new Iterator. This capability is a big improvement over the one-shot iterator presented in Listing 4–7. Interfaces Within Classes Interfaces can be nested within classes. Once declared, an interface is considered to be static, even if it is not declared static. For example, Listing 4–15 declares an enclosing class named X along with two nested static interfaces named A and B. Listing 4–15. Declaring a pair of interfaces within a class class X { interface A { } static interface B { } } As with nested classes, nested interfaces help to implement top-level class architecture by being implemented by nested classes. Collectively, these types are nested because they cannot (as in Listing 4–13’s Iter local class) or need not appear at the same level as a top-level class and pollute its package namespace. NOTE: The previous chapter’s introduction to interfaces showed you how to declare constants and method headers in the body of an interface. You can also declare interfaces and classes in an interface’s body. Because there does not appear to be a good reason to do this, it is probably best to avoid nesting interfaces and/or classes within interfaces. CHAPTER 4: Mastering Advanced Language Features Part 1 151 Packages Hierarchical structures organize items in terms of hierarchical relationships that exist between those items. For example, a filesystem might contain a taxes directory with multiple year subdirectories, where each subdirectory contains tax information pertinent to that year. Also, an enclosing class might contain multiple nested classes that only make sense in the context of the enclosing class. Hierarchical structures also help to avoid name conflicts. For example, two files cannot have the same name in a nonhierarchical filesystem (which consists of a single directory). In contrast, a hierarchical filesystem lets same-named files exist in different directories. Similarly, two enclosing classes can contain same-named nested classes. Name conflicts do not exist because items are partitioned into different namespaces. Java also supports the partitioning of top-level types into multiple namespaces, to better organize these types and to also prevent name conflicts. Java uses packages to accomplish these tasks. This section introduces you to packages. After defining this term and explaining why package names must be unique, the section presents the package and import statements. It next explains how the virtual machine searches for packages and types, and then presents an example that shows you how to work with packages. This section closes by showing you how to encapsulate a package of classfiles into JAR files. What Are Packages? A package is a unique namespace that can contain a combination of top-level classes, other top-level types, and subpackages. Only types that are declared public can be accessed from outside the package. Furthermore, the constants, constructors, methods, and nested types that describe a class’s interface must be declared public to be accessible from beyond the package. Every package has a name, which must be a nonreserved identifier. The member access operator separates a package name from a subpackage name, and separates a package or subpackage name from a type name. For example, the two member access operators in graphics.shapes.Circle separate package name graphics from the shapes subpackage name, and separate subpackage name shapes from the Circle type name. NOTE: Each of Java SE’s standard class library and Android’s class library organizes its many classes and other top-level types into multiple packages. Many of these packages are subpackages of the standard java package. Examples include java.io (types related to input/output operations), java.lang (language-oriented types), java.lang.reflect (reflection-oriented language types), java.net (network-oriented types), and java.util (utility types). CHAPTER 4: Mastering Advanced Language Features Part 1 152 Package Names Must Be Unique Suppose you have two different graphics.shapes packages, and suppose that each shapes subpackage contains a Circle class with a different interface. When the compiler encounters System.out.println(new Circle(10.0, 20.0, 30.0).area()); in the source code, it needs to verify that the area() method exists. The compiler will search all accessible packages until it finds a graphics.shapes package that contains a Circle class. If the found package contains the appropriate Circle class with an area() method, everything is fine. Otherwise, if the Circle class does not have an area() method, the compiler will report an error. This scenario illustrates the importance of choosing unique package names. Specifically, the top-level package name must be unique. The convention in choosing this name is to take your Internet domain name and reverse it. For example, I would choose ca.mb.javajeff as my top-level package name because javajeff.mb.ca is my domain name. I would then specify ca.mb.javajeff.graphics.shapes.Circle to access Circle. NOTE: Reversed Internet domain names are not always valid package names. One or more of its component names might start with a digit (6.com), contain a hyphen (-) or other illegal character (aq-x.com), or be one of Java’s reserved words (int.com). Convention dictates that you prefix the digit with an underscore (com._6), replace the illegal character with an underscore (com.aq_x), and suffix the reserved word with an underscore (com.int_). The Package Statement The package statement identifies the package in which a source file’s types are located. This statement consists of reserved word package, followed by a member access operator–separated list of package and subpackage names, followed by a semicolon. For example, package graphics; specifies that the source file’s types locate in a package named graphics, and package graphics.shapes; specifies that the source file’s types locate in the graphics package’s shapes subpackage. By convention, a package name is expressed in lowercase. If the name consists of multiple words, each word except for the first word is capitalized. Only one package statement can appear in a source file. When it is present, nothing apart from comments must precede this statement. CAUTION: Specifying multiple package statements in a source file or placing anything apart from comments above a package statement causes the compiler to report an error. Java implementations map package and subpackage names to same-named directories. For example, an implementation would map graphics to a directory named CHAPTER 4: Mastering Advanced Language Features Part 1 153 graphics, and would map graphics.shapes to a shapes subdirectory of graphics. The Java compiler stores the classfiles that implement the package’s types in the corresponding directory. NOTE: If a source file does not contain a package statement, the source file’s types are said to belong to the unnamed package. This package corresponds to the current directory. The Import Statement Imagine having to repeatedly specify ca.mb.javajeff.graphics.shapes.Circle or some other lengthy package-qualified type name for each occurrence of that type in source code. Java provides an alternative that lets you avoid having to specify package details. This alternative is the import statement. The import statement imports types from a package by telling the compiler where to look for unqualified type names during compilation. This statement consists of reserved word import, followed by a member access operator–separated list of package and subpackage names, followed by a type name or * (asterisk), followed by a semicolon. The * symbol is a wildcard that represents all unqualified type names. It tells the compiler to look for such names in the import statement’s specified package, unless the type name is found in a previously searched package. For example, import ca.mb.javajeff.graphics.shapes.Circle; tells the compiler that an unqualified Circle class exists in the ca.mb.javajeff.graphics.shapes package. Similarly, import ca.mb.javajeff.graphics.shapes.*; tells the compiler to look in this package if it encounters a Rectangle class, a Triangle class, or even an Employee class (if Employee has not already been found). TIP: You should avoid using the * wildcard so that other developers can easily see which types are used in source code. Because Java is case sensitive, package and subpackage names specified in an import statement must be expressed in the same case as that used in the package statement. When import statements are present in source code, only a package statement and comments can precede them. CAUTION: Placing anything other than a package statement, import/static import statements, and comments above an import statement causes the compiler to report an error. You can run into name conflicts when using the wildcard version of the import statement because any unqualified type name matches the wildcard. For example, you have graphics.shapes and geometry packages that each contain a Circle class, the source CHAPTER 4: Mastering Advanced Language Features Part 1 154 code begins with import geometry.*; and import graphics.shape.*; statements, and it also contains an unqualified occurrence of Circle. Because the compiler does not know if Circle refers to geometry’s Circle class or graphics.shape’s Circle class, it reports an error. You can fix this problem by qualifying Circle with the correct package name. NOTE: The compiler automatically imports the String class and other types from the java.lang package, which is why it is not necessary to qualify String with java.lang. Searching for Packages and Types Newcomers to Java who first start to work with packages often become frustrated by “no class definition found” and other errors. This frustration can be partly avoided by understanding how the virtual machine searches for packages and types. This section explains how the search process works. To understand this process, you need to realize that the compiler is a special Java application that runs under the control of the virtual machine. Furthermore, there are two different forms of search. Compile-Time Search When the compiler encounters a type expression (such as a method call) in source code, it must locate that type’s declaration to verify that the expression is legal (a method exists in the type’s class whose parameter types match the types of the arguments passed in the method call, for example). The compiler first searches the Java platform packages (which contain class library types). It then searches extension packages (for extension types). If the -sourcepath command-line option was specified when starting the virtual machine (via javac), the compiler searches the indicated path’s source files. NOTE: Java platform packages are stored in rt.jar and a few other important JAR files. Extension packages are stored in a special extensions directory named ext. Otherwise, it searches the user classpath (in left-to-right order) for the first user classfile or source file containing the type. If no user classpath is present, the current directory is searched. If no package matches or the type still cannot be found, the compiler reports an error. Otherwise, the compiler records the package information in the classfile. NOTE: The user classpath is specified via the -classpath option used to start the virtual machine or, if not present, the CLASSPATH environment variable. This book was purchased by harshamv@variable3.comCHAPTER 4: Mastering Advanced Language Features Part 1 155 Runtime Search When the compiler or any other Java application runs, the virtual machine will encounter types and must load their associated classfiles via special code known as a classloader. It will use the previously stored package information that is associated with the encountered type in a search for that type’s classfile. The virtual machine searches the Java platform packages, followed by extension packages, followed by the user classpath (in left-to-right order) for the first classfile that contains the type. If no user classpath is present, the current directory is searched. If no package matches or the type cannot be found, a “no class definition found” error is reported. Otherwise, the classfile is loaded into memory. NOTE: Whether you use the -classpath option or the CLASSPATH environment variable to specify a user classpath, there is a specific format that must be followed. Under Windows, this format is expressed as path1;path2;..., where path1, path2, and so on are the locations of package directories. Under Unix and Linux, this format changes to path1:path2:.... Playing with Packages Suppose your application needs to log messages to the console, to a file, or to another destination (perhaps to an application running on another computer). Furthermore, suppose the application needs to perform some combination of these tasks. To demonstrate packages, this section presents a simple and reusable logging library. This library consists of an interface named Logger, an abstract class named LoggerFactory, and a pair of package-private classes named Console and File. NOTE: The logging library is an example of the Abstract Factory design pattern, which is presented on page 87 of Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (Addison-Wesley, 1995; ISBN: 0201633612). Listing 4–16 presents the Logger interface, which describes objects that log messages. Listing 4–16. Describing objects that log messages via the Logger interface package logging; public interface Logger { boolean connect(); boolean disconnect(); boolean log(String msg); } CHAPTER 4: Mastering Advanced Language Features Part 1 156 Each of the connect(), disconnect(), and log() methods returns true upon success, and false upon failure. (Later in this chapter, you will discover a better technique for dealing with failure.) Listing 4–17 presents the LoggerFactory abstract class. Listing 4–17. Obtaining a logger for logging messages to a specific destination package logging; public abstract class LoggerFactory { public final static int CONSOLE = 0; public final static int FILE = 1; public static Logger newLogger(int dstType, String...dstName) { switch (dstType) { case CONSOLE: return new Console(dstName.length == 0 ? null : dstName[0]); case FILE : return new File(dstName.length == 0 ? null : dstName[0]); default : return null; } } } newLogger() returns a Logger for logging messages to an appropriate destination. It uses the variable arguments feature to optionally accept an extra String argument for those destination types that require the argument. For example, FILE requires a filename. Listing 4–18 presents the package-private Console class. Listing 4–18. Logging messages to the console package logging; class Console implements Logger { private String dstName; Console(String dstName) { this.dstName = dstName; } public boolean connect() { return true; } public boolean disconnect() { return true; } public boolean log(String msg) { System.out.println(msg); return true; } } CHAPTER 4: Mastering Advanced Language Features Part 1 157 Console’s package-private constructor saves its argument, which most likely will be null because there is no need for a String argument. Perhaps a future version of Console will use this argument to identify one of multiple console windows. Listing 4–19 presents the package-private File class. Listing 4–19. Logging messages to a file (eventually) package logging; class File implements Logger { private String dstName; File(String dstName) { this.dstName = dstName; } public boolean connect() { if (dstName == null) return false; System.out.println("opening file " + dstName); return true; } public boolean disconnect() { if (dstName == null) return false; System.out.println("closing file " + dstName); return true; } public boolean log(String msg) { if (dstName == null) return false; System.out.println("writing " + msg + " to file " + dstName); return true; } } Unlike Console, File requires a nonnull argument. Each method first verifies that this argument is not null. If the argument is null, the method returns false to signify failure. (In Chapter 10, I refactor File to incorporate appropriate file-writing code.) The logging library allows us to introduce portable logging code into an application. Apart from a call to newLogger(), this code will remain the same regardless of the logging destination. Listing 4–20 presents an application that tests this library. Listing 4–20. Testing the logging library import logging.*; public class TestLogger { public static void main(String[] args) { Logger logger = LoggerFactory.newLogger(LoggerFactory.CONSOLE); if (logger.connect()) CHAPTER 4: Mastering Advanced Language Features Part 1 158 { logger.log("test message #1"); logger.disconnect(); } else System.out.println("cannot connect to console-based logger"); logger = LoggerFactory.newLogger(LoggerFactory.FILE, "x.txt"); if (logger.connect()) { logger.log("test message #2"); logger.disconnect(); } else System.out.println("cannot connect to file-based logger"); logger = LoggerFactory.newLogger(LoggerFactory.FILE); if (logger.connect()) { logger.log("test message #3"); logger.disconnect(); } else System.out.println("cannot connect to file-based logger"); } } Follow the steps (which assume that the JDK has been installed) to create the logging package and TestLogger application, and to run this application: 1. Create a new directory and make this directory current. 2. Create a logging directory in the current directory. 3. Copy Listing 4–16 to a file named Logger.java in the logging directory. 4. Copy Listing 4–17 to a file named LoggerFactory.java in the logging directory. 5. Copy Listing 4–18 to a file named Console.java in the logging directory. 6. Copy Listing 4–19 to a file named File.java in the logging directory. 7. Copy Listing 4–20 to a file named TestLogger.java in the current directory. 8. Execute javac TestLogger.java, which also compiles logger’s source files. 9. Execute java TestLogger. After completing the previous step, you should observe the following output from the TestLogger application: test message #1 opening file x.txt writing test message #2 to file x.txt closing file x.txt cannot connect to file-based logger CHAPTER 4: Mastering Advanced Language Features Part 1 159 What happens when logging is moved to another location? For example, move logging to the root directory and run TestLogger. You will now observe an error message about the virtual machine not finding the logging package and its LoggerFactory classfile. You can solve this problem by specifying -classpath when running the java tool, or by adding the location of the logging package to the CLASSPATH environment variable. For example, I chose to use -classpath in the following Windows-specific command line: java -classpath \;. TestLogger The backslash represents the root directory in Windows. (I could have specified a forward slash as an alternative.) Also, the period represents the current directory. If it is missing, the virtual machine complains about not finding the TestLogger classfile. TIP: If you discover an error message where the virtual machine reports that it cannot find an application classfile, try appending a period character to the classpath. Doing so will probably fix the problem. Packages and JAR Files Chapter 1 introduced you to the Java SDK’s jar tool, which is used to archive classfiles in JAR files, and is also used to extract a JAR file’s classfiles. It probably comes as no surprise that you can store packages in JAR files, which greatly simplify the distribution of your package-based class libraries. NOTE: Java version 7 will introduce modules as a replacement to JAR files. Modules address JAR file problems such as a lack of versioning support; no reliable way to express, resolve, and enforce one JAR file’s dependency on another JAR file; and having to specify a JAR file as part of the classpath. Because the JAR file’s location might change during deployment, developers are forced to correct all references to the JAR file. This new feature will probably include a new reserved word named module. To show you how easy it is to store a package in a JAR file, we will create a logger.jar file that contains the logging package’s four classfiles (Logger.class, LoggerFactory.class, Console.class, and File.class). Complete the following steps to accomplish this task: 1. Make sure that the current directory contains the previously created logging directory with its four classfiles. 2. Execute jar cf logger.jar logging\*.class. You could alternatively execute jar cf logger.jar logging/*.class. You should now find a logger.jar file in the current directory. To prove to yourself that this file contains the four classfiles, execute jar tf logger.jar. CHAPTER 4: Mastering Advanced Language Features Part 1 160 You can run TestLogger.class by adding logger.jar to the classpath. For example, you can run TestLogger under Windows via java -classpath logger.jar;. TestLogger. Static Imports An interface should only be used to declare a type. However, some developers violate this principle by using interfaces to only export constants. Such interfaces are known as constant interfaces, and Listing 4–21 presents an example. Listing 4–21. Declaring a constant interface public interface Directions { int NORTH = 0; int SOUTH = 1; int EAST = 2; int WEST = 3; } Developers who resort to constant interfaces do so to avoid having to prefix a constant’s name with the name of its class (as in Math.PI, where PI is a constant in the java.lang.Math class). They do this by implementing the interface—see Listing 4–22. Listing 4–22. Implementing a constant interface public class TrafficFlow implements Directions { public static void main(String[] args) { showDirection((int)(Math.random()*4)); } private static void showDirection(int dir) { switch (dir) { case NORTH: System.out.println("Moving north"); break; case SOUTH: System.out.println("Moving south"); break; case EAST : System.out.println("Moving east"); break; case WEST : System.out.println("Moving west"); } } } Listing 4–22’s TrafficFlow class implements Directions for the sole purpose of not having to specify Directions.NORTH, Directions.SOUTH, Directions.EAST, and Directions.WEST. This is an appalling misuse of an interface. These constants are nothing more than an implementation detail that should not be allowed to leak into the class’s exported interface, because they might confuse the class’s users (what is the purpose of these constants?). Also, they represent a future commitment: even when the class no longer uses these constants, the interface must remain to ensure binary compatibility. CHAPTER 4: Mastering Advanced Language Features Part 1 161 Java version 5 introduced an alternative that satisfies the desire for constant interfaces while avoiding their problems. This static imports feature lets you import a class’s static members so that you do not have to qualify them with their class names. It is implemented via a small modification to the import statement, as follows: import static packagespec . classname . ( staticmembername | * ); The static import statement specifies static after import. It then specifies a member access operator–separated list of package and subpackage names, which is followed by the member access operator and a class’s name. Once again, the member access operator is specified, followed by a single static member name or the asterisk wildcard. CAUTION: Placing anything apart from a package statement, import/static import statements, and comments above a static import statement causes the compiler to report an error. You specify a single static member name to import only that name: import static java.lang.Math.PI; // Import the PI static field only. import static java.lang.Math.cos; // Import the cos() static method only. In contrast, you specify the wildcard to import all static member names: import static java.lang.Math.*; // Import all static members from Math. You can now refer to the static member(s) without having to specify the class name: System.out.println(cos(PI)); Using multiple static import statements can result in name conflicts, which causes the compiler to report errors. For example, suppose your geom package contains a Circle class with a static member named PI. Now suppose you specify import static java.lang.Math.*; and import static geom.Circle.*; at the top of your source file. Finally, suppose you specify System.out.println(PI); somewhere in that file’s code. The compiler reports an error because it does not know if PI belongs to Math or Circle. Exceptions In an ideal world, nothing bad ever happens when an application runs. For example, a file always exists when the application needs to open the file, the application is always able to connect to a remote computer, and the virtual machine never runs out of memory when the application needs to instantiate objects. In contrast, real-world applications occasionally attempt to open files that do not exist, attempt to connect to remote computers that are unable to communicate with them, and require more memory than the virtual machine can provide. Your goal is to write code that properly responds to these and other exceptional situations (exceptions). This section introduces you to exceptions. After defining this term, the section looks at representing exceptions in source code. It then examines the topics of throwing and CHAPTER 4: Mastering Advanced Language Features Part 1 162 handling exceptions, and concludes by discussing how to perform cleanup tasks before a method returns, whether or not an exception has been thrown. What Are Exceptions? An exception is a divergence from an application’s normal behavior. For example, the application attempts to open a nonexistent file for reading. The normal behavior is to successfully open the file and begin reading its contents. However, the file cannot be read if the file does not exist. This example illustrates an exception that cannot be prevented. However, a workaround is possible. For example, the application can detect that the file does not exist and take an alternate course of action, which might include telling the user about the problem. Unpreventable exceptions where workarounds are possible must not be ignored. Exceptions can occur because of poorly written code. For example, an application might contain code that accesses each element in an array. Because of careless oversight, the array-access code might attempt to access a nonexistent array element, which leads to an exception. This kind of exception is preventable by writing correct code. Finally, an exception might occur that cannot be prevented, and for which there is no workaround. For example, the virtual machine might run out of memory, or perhaps it cannot find a classfile. This kind of exception, known as an error, is so serious that it is impossible (or at least inadvisable) to work around; the application must terminate, presenting a message to the user that states why it is terminating. Representing Exceptions in Source Code An exception can be represented via error codes or objects. This section discusses each kind of representation and explains why objects are superior. It then introduces you to Java’s exception and error class hierarchy, emphasizing the difference between checked and runtime exceptions. It closes by discussing custom exception classes. Error Codes Versus Objects One way to represent exceptions in source code is to use error codes. For example, a method might return true on success and false when an exception occurs. Alternatively, a method might return 0 on success and a nonzero integer value that identifies a specific kind of exception. Developers traditionally designed methods to return error codes; I demonstrated this tradition in each of the three methods in Listing 4–16’s Logger interface. Each method returns true on success, or returns false to represent an exception (unable to connect to the logger, for example). Although a method’s return value must be examined to see if it represents an exception, error codes are all too easy to ignore. For example, a lazy developer might ignore the CHAPTER 4: Mastering Advanced Language Features Part 1 163 return code from Logger’s connect() method and attempt to call log(). Ignoring error codes is one reason why a new approach to dealing with exceptions has been invented. This new approach is based on objects. When an exception occurs, an object representing the exception is created by the code that was running when the exception occurred. Details describing the exception’s surrounding context are stored in the object. These details are later examined to work around the exception. The object is then thrown, or handed off to the virtual machine to search for a handler, code that can handle the exception. (If the exception is an error, the application should not provide a handler.) When a handler is located, its code is executed to provide a workaround. Otherwise, the virtual machine terminates the application. Apart from being too easy to ignore, an error code’s Boolean or integer value is less meaningful than an object name. For example, fileNotFound is self-evident, but what does false mean? Also, an object can contain information about what led to the exception. These details can be helpful to a suitable workaround. The Throwable Class Hierarchy Java provides a hierarchy of classes that represent different kinds of exceptions. These classes are rooted in java.lang.Throwable, the ultimate superclass for all throwables (exception and error objects—exceptions and errors, for short—that can be thrown). Table 4–1 identifies and describes most of Throwable’s constructors and methods. Table 4–1. Throwable’s Constructors and Methods Method Description Throwable() Create a throwable with a null detail message and cause. Throwable(String message) Create a throwable with the specified detail message and a null cause. Throwable(String message, Throwable cause) Create a throwable with the specified detail message and cause. Throwable(Throwable cause) Create a throwable whose detail message is the string representation of a nonnull cause, or null. Throwable getCause() Return the cause of this throwable. If there is no cause, null is returned. String getMessage() Return this throwable’s detail message, which might be null. StackTraceElement[] getStackTrace() Provide programmatic access to the stack trace information printed by printStackTrace() as an array of stack trace elements, each representing one stack frame. CHAPTER 4: Mastering Advanced Language Features Part 1 164 Method Description Throwable initCause(Throwable cause) Initialize the cause of this throwable to the specified value. void printStackTrace() Print this throwable and its backtrace of stack frames to the standard error stream. It is not uncommon for a class’s public methods to call helper methods that throw various exceptions. A public method will probably not document exceptions thrown from a helper method because they are implementation details that often should not be visible to the public method’s caller. However, because this exception might be helpful in diagnosing the problem, the public method can wrap the lower-level exception in a higher-level exception that is documented in the public method’s contract interface. The wrapped exception is known as a cause because its existence causes the higher-level exception to be thrown. When an exception is thrown, it leaves behind a stack of unfinished method calls. Each stack entry is represented by an instance of the java.lang.StackTraceElement class. This class’s methods provide access to information about a stack entry. For example, public String getMethodName() returns the name of an unfinished method. Moving down the throwable hierarchy, you encounter the java.lang.Exception and java.lang.Error classes, which respectively represent exceptions and errors. Each class offers four constructors that pass their arguments to their Throwable counterparts, but provides no methods apart from those that are inherited from Throwable. Exception is itself subclassed by java.lang.CloneNotSupportedException (discussed in Chapter 3), java.lang.IOException (discussed in Chapter 10), and other classes. Similarly, Error is itself subclassed by java.lang.AssertionError (discussed in Chapter 5), java.lang.OutOfMemoryError, and other classes. CAUTION: Never instantiate Throwable, Exception, or Error. The resulting objects are meaningless because they are too generic. Checked Exceptions Versus Runtime Exceptions A checked exception is an exception that represents a problem with the possibility of recovery, and for which the developer must provide a workaround. The developer checks (examines) the code to ensure that the exception is handled in the method where it is thrown, or is explicitly identified as being handled elsewhere. Exception and all subclasses except for RuntimeException (and its subclasses) describe checked exceptions. For example, the aforementioned CloneNotSupportedException and IOException classes describe checked exceptions. (CloneNotSupportedException should not be checked because there is no runtime workaround for this kind of exception.) This book was purchased by harshamv@variable3.comCHAPTER 4: Mastering Advanced Language Features Part 1 165 A runtime exception is an exception that represents a coding mistake. This kind of exception is also known as an unchecked exception because it does not need to be handled or explicitly identified—the mistake must be fixed. Because these exceptions can occur in many places, it would be burdensome to be forced to handle them. RuntimeException and its subclasses describe unchecked exceptions. For example, java.lang.ArithmeticException describes arithmetic problems such as integer division by zero. Another example is java.lang.ArrayIndexOutOfBoundsException. (In hindsight, RuntimeException should have been named UncheckedException because all exceptions occur at runtime.) NOTE: Many developers are not happy with checked exceptions because of the work involved in having to handle them. This problem is made worse by libraries providing methods that throw checked exceptions when they should throw unchecked exceptions. As a result, many modern languages support only unchecked exceptions. Custom Exception Classes You can declare your own exception classes. Before doing so, ask yourself if an existing exception class in Java’s class library meets your needs. If you find a suitable class, you should reuse it. (Why reinvent the wheel?) Other developers will already be familiar with the existing class, and this knowledge will make your code easier to learn. If no existing class meets your needs, think about whether to subclass Exception or RuntimeException. In other words, will your exception class be checked or unchecked? As a rule of thumb, your class should subclass RuntimeException if you think that it will describe a coding mistake. TIP: When you name your class, follow the convention of providing an Exception suffix. This suffix clarifies that your class describes an exception. Suppose you are creating a Media class whose static methods perform various media- oriented utility tasks. For example, one method converts files in non-MP3 media formats to MP3 format. This method will be passed source file and destination file arguments, and will convert the source file to the format implied by the destination file’s extension. Before performing the conversion, the method needs to verify that the source file’s format agrees with the format implied by its file extension. If there is no agreement, an exception must be thrown. Furthermore, this exception must store the expected and existing media formats so that a handler can identify them in a message to the user. Because Java’s class library does not provide a suitable exception class, you decide to introduce a class named InvalidMediaFormatException. Detecting an invalid media format is not the result of a coding mistake, and so you also decide to extend Exception to indicate that the exception is checked. Listing 4–23 presents this class’s declaration. CHAPTER 4: Mastering Advanced Language Features Part 1 166 Listing 4–23. Declaring a custom exception class public class InvalidMediaFormatException extends Exception { private String expectedFormat; private String existingFormat; public InvalidMediaFormatException(String expectedFormat, String existingFormat) { super("Expected format: " + expectedFormat + ", Existing format: " + existingFormat); this.expectedFormat = expectedFormat; this.existingFormat = existingFormat; } public String getExpectedFormat() { return expectedFormat; } public String getExistingFormat() { return existingFormat; } } InvalidMediaFormatException provides a constructor that calls Exception’s public Exception(String message) constructor with a detail message that includes the expected and existing formats. It is wise to capture such details in the detail message because the problem that led to the exception might be hard to reproduce. InvalidMediaFormatException also provides getExpectedFormat() and getExistingFormat() methods that return these formats. Perhaps a handler will present this information in a message to the user. Unlike the detail message, this message might be localized, expressed in the user’s language (French, German, English, and so on). Throwing Exceptions Now that you have created an InvalidMediaFormatException class, you can declare the Media class and begin to code its convert() method. The initial version of this method validates its arguments, and then verifies that the source file’s media format agrees with the format implied by its file extension. Check out Listing 4–24. Listing 4–24. Throwing exceptions from the convert() method public static void convert(String srcName, String dstName) throws InvalidMediaFormatException { if (srcName == null) throw new NullPointerException(srcName + " is null"); if (dstName == null) throw new NullPointerException(dstName + " is null"); // Code to access source file and verify that its format matches the // format implied by its file extension. // // Assume that the source file's extension is RM (for Real Media) and // that the file's internal signature suggests that its format is CHAPTER 4: Mastering Advanced Language Features Part 1 167 // Microsoft WAVE. String expectedFormat = "RM"; String existingFormat = "WAVE"; throw new InvalidMediaFormatException(expectedFormat, existingFormat); } Listing 4–24 demonstrates a throws clause, which consists of reserved word throws followed by a comma-separated list of checked exception class names, and which is appended to a method header. This clause identifies all checked exceptions that are thrown out of the method, and which must be handled by some other method. Listing 4–24 also demonstrates the throw statement, which consists of reserved word throw followed by an instance of Throwable or a subclass. (You typically instantiate an Exception subclass.) This statement throws the instance to the virtual machine, which then searches for a suitable handler to handle the exception. The first use of the throw statement is to throw a java.lang.NullPointerException instance when a null reference is passed as the source or destination filename. This unchecked exception is commonly thrown to indicate that a contract has been violated via a passed null reference. For example, you cannot pass null filenames to convert(). The second use of the throw statement is to throw an InvalidMediaFormatException instance when the expected media format does not match the existing format. In the contrived example, the exception is thrown because the expected format is RM and the existing format is WAVE. Unlike InvalidMediaFormatException, NullPointerException is not listed in convert()’s throws clause because NullPointerException instances are unchecked. They can occur so frequently that it is too big a burden to force the developer to properly handle these exceptions. Instead, the developer should write code that minimizes their occurrences. NullPointerException is one kind of exception that is thrown when an argument proves to be invalid. The java.lang.IllegalArgumentException class generalizes the illegal argument scenario to include other kinds of illegal arguments. For example, Listing 4–25 throws an IllegalArgumentException instance when a numeric argument is negative. Listing 4–25. Throwing an IllegalArgumentException instance when x is negative (you can’t calculate a negative number’s square root) public static double sqrt(double x) { if (x < 0) throw new IllegalArgumentException(x + " is negative"); // Calculate the square root of x. } There are a few additional items to keep in mind when working with throws clauses and throw statements:  You can append a throws clause to a constructor and throw an exception from the constructor when something goes wrong while the constructor is executing. The resulting object will not be created. CHAPTER 4: Mastering Advanced Language Features Part 1 168  When an exception is thrown out of an application’s main() method, the virtual machine terminates the application and calls the exception’s printStackTrace() method to print, to the console, the sequence of nested method calls that was awaiting completion when the exception was thrown.  If a superclass method declares a throws clause, the overriding subclass method does not have to declare a throws clause. However, if it does declare a throws clause, the clause must not include the names of exception classes that are not also included in the superclass method’s throws clause.  A checked exception class name does not need to appear in a throws clause when the name of its superclass appears.  The compiler reports an error when a method throws a checked exception and does not also handle the exception or list the exception in its throws clause.  Do not include the names of unchecked exception classes in a throws clause. These names are not required because such exceptions should never occur. Furthermore, they only clutter source code, and possibly confuse someone who is trying to understand that code.  You can declare a checked exception class name in a method’s throws clause without throwing an instance of this class from the method. Perhaps the method has yet to be fully coded. Handling Exceptions A method indicates its intention to handle one or more exceptions by specifying a try statement and one or more appropriate catch clauses. The try statement consists of reserved word try followed by a brace-delimited body. You place code that throws exceptions into this body. A catch clause consists of reserved word catch, followed by a round bracket–delimited single-parameter list that specifies an exception class name, followed by a brace- delimited body. You place code that handles exceptions whose types match the type of the catch clause’s parameter list’s exception class parameter in this body. A catch clause is specified immediately after a try statement’s body. When an exception is thrown, the virtual machine searches for a handler by first examining the catch clause to see if its parameter type matches or is the superclass type of the exception that has been thrown. If the catch clause is found, its code executes and the exception is handled. Otherwise, the virtual machine proceeds up the method-call stack, looking for the first method whose try statement contains an appropriate catch clause. This process continues unless a catch clause is found or execution leaves the main() method. CHAPTER 4: Mastering Advanced Language Features Part 1 169 Listing 4–26 illustrates try and catch. Listing 4–26. Handling a thrown exception public static void main(String[] args) { if (args.length != 2) { System.err.println("usage: java Converter srcfile dstfile"); return; } try { Media.convert(args[0], args[1]); } catch (InvalidMediaFormatException imfe) { System.out.println("Unable to convert " + args[0] + " to " + args[1]); System.out.println("Expecting " + args[0] + " to conform to " + imfe.getExpectedFormat() + " format."); System.out.println("However, " + args[0] + " conformed to " + imfe.getExistingFormat() + " format."); } } Media’s convert() method is placed in a try statement’s body because this method is capable of throwing an instance of the checked InvalidMediaFormatException class— checked exceptions must be handled or be declared to be thrown via a throws clause that is appended to the method. A catch clause immediately follows try’s body. This clause presents a parameter list whose single parameter matches the type of the InvalidMediaFormatException object thrown from convert(). When the object is thrown, the virtual machine will transfer execution to the statements within this clause. TIP: You might want to name your catch clause parameters using the abbreviated style shown in Listing 4–26. Not only does this convention result in more meaningful exception-oriented parameter names (imfe indicates that an InvalidMediaFormatException has been thrown), it will probably reduce compiler errors. It is common practice to name a catch clause’s parameter e, for convenience. (Why type a long name?) However, the compiler will report an error when a previously declared local variable or parameter also uses e as its name—multiple same-named local variables and parameters cannot exist in the same scope. The catch clause’s statements are designed to provide a descriptive error message to the user. A more sophisticated application would localize these names so that the user could read the message in the user’s language. The developer-oriented detail message is not output because it is not necessary in this trivial application. CHAPTER 4: Mastering Advanced Language Features Part 1 170 NOTE: A developer-oriented detail message is typically not localized. Instead, it is expressed in the developer’s language. Users should never see detail messages. You can specify multiple catch clauses after try’s body. For example, a later version of convert() will also throw java.io.FileNotFoundException when it cannot open the source file or create the destination file, and IOException when it cannot read from the source file or write to the destination file. All of these exceptions must be handled. Listing 4–27 illustrates multiple catch clauses. Listing 4–27. Handling more than one thrown exception try { Media.convert(args[0], args[1]); } catch (InvalidMediaFormatException imfe) { System.out.println("Unable to convert " + args[0] + " to " + args[1]); System.out.println("Expecting " + args[0] + " to conform to " + imfe.getExpectedFormat() + " format."); System.out.println("However, " + args[0] + " conformed to " + imfe.getExistingFormat() + " format."); } catch (FileNotFoundException fnfe) { } catch (IOException ioe) { } Listing 4–27 assumes that convert() also throws IOException and FileNotFoundException. Although this assumption suggests that both classes need to be listed in convert()’s throws clause, only IOException needs to be listed because it is the superclass of FileNotFoundException. CAUTION: The compiler reports an error when you specify two or more catch clauses with the same parameter type after a try body. Example: try {} catch (IOException ioe1) {} catch (IOException ioe2) {}. You must merge these catch clauses into one clause. Catch clauses often can be specified in any order. However, the compiler restricts this order when one catch clause’s parameter is a supertype of another catch clause’s parameter. The subtype parameter catch clause must precede the supertype parameter catch clause; otherwise, the subtype parameter catch clause will never be called. For example, the FileNotFoundException catch clause must precede the IOException catch clause. If the compiler allowed the IOException catch clause to be specified first, the FileNotFoundException catch clause would never execute because a FileNotFoundException instance is also an instance of its IOException superclass. CHAPTER 4: Mastering Advanced Language Features Part 1 171 NOTE: Java version 7 introduces a catch clause improvement known as multicatch, which lets you place common exception-handling code in a single catch clause. For example, catch (InvalidMediaFormatException | UnsupportedMediaFormatException ex) { /* common code */ } handles InvalidMediaFormatException and a similar UnsupportedMediaFormatException in one place. Multicatch is not always necessary. For example, you do not need to specify catch (FileNotFoundException | IOException exc) { /* suitable common code */ } to handle FileNotFoundException and IOException because catch (IOException ioe) accomplishes the same task, by catching FileNotFoundException as well as IOException. The empty FileNotFoundException and IOException catch clauses illustrate the often- seen problem of leaving catch clauses empty because they are inconvenient to code. Unless you have a good reason, do not create an empty catch clause. It swallows exceptions and you do not know that the exceptions were thrown. CAUTION: Do not code empty catch clauses. Because they swallow exceptions, you will probably find it more difficult to debug a faulty application. While discussing the Throwable class, I discussed wrapping lower-level exceptions in higher-level exceptions. This activity will typically take place in a catch clause, and is illustrated in Listing 4–28. Listing 4–28. Throwing a new exception that contains a wrapped exception catch (IOException ioe) { throw new ReportCreationException(ioe); } This example assumes that a helper method has just thrown a generic IOException as the result of trying to create a report. The public method’s contract states that ReportCreationException is thrown in this case. To satisfy the contract, the latter exception is thrown. To satisfy the developer who is responsible for debugging a faulty application, the IOException instance is wrapped inside the ReportCreationException instance that is thrown to the public method’s caller. Sometimes, a catch clause might not be able to fully handle an exception. Perhaps it needs access to information provided by some ancestor method in the method-call stack. However, the catch clause might be able to partly handle the exception. In this case, it should partly handle the exception, and then rethrow the exception so that a handler in the ancestor method can finish handling the exception. This scenario is demonstrated in Listing 4–29. CHAPTER 4: Mastering Advanced Language Features Part 1 172 Listing 4–29. Rethrowing an exception catch (FileNotFoundException fnfe) { // Provide code to partially handle the exception here. throw fnfe; // Rethrow the exception here. } NOTE: Java version 7 introduces a catch clause improvement known as final rethrow, which lets you declare a catch clause parameter final in order to throw only those checked exception types that were thrown in the try body, are a subtype of the catch parameter type, and are not caught in preceding catch clauses. For example, suppose you declare the following method: void method() throws Exc1, Exc2 // Exc1 and Exc2 extend Exception { try { /* Code that can throw Exc1,Exc2 */ } catch (Exception exc) { logger.log(exc); throw exc; // Attempt to throw caught exception as an Exception } } The compiler would report an error when asked to compile this method because you are trying to rethrow an exception that is first upcasted to Exception, but Exception is not listed in method()’s throws clause. However, if you change the catch clause header to catch (final Exception exc), the compiler will not report an error because you are rethrowing Exc1 or Exc2 exceptions without the upcasting. Performing Cleanup In some situations, you might want to prevent an exception from being thrown out of a method before the method’s cleanup code is executed. For example, you might want to close a file that was opened, but could not be written, possibly because of insufficient disk space. Java provides the finally clause for this situation. The finally clause consists of reserved word finally followed by a body, which provides the cleanup code. A finally clause follows either a catch clause or a try body. In the former case, the exception is handled (and possibly rethrown) before finally executes. In the latter case, finally executes before the exception is thrown and handled. CHAPTER 4: Mastering Advanced Language Features Part 1 173 Listing 4–30 demonstrates the finally clause in the context of a file-copying application’s main() method. Listing 4–30. Cleaning up after handling a thrown exception public static void main(String[] args) { if (args.length != 2) { System.err.println("usage: java Copy srcfile dstfile"); return; } FileInputStream fis = null; try { fis = new FileInputStream(args[0]); FileOutputStream fos = null; try { fos = new FileOutputStream(args[1]); int b; // I chose b instead of byte because byte is a reserved word. while ((b = fis.read()) != -1) fos.write(b); } catch (FileNotFoundException fnfe) { String msg = args[1] + " could not be created, possibly because " + "it might be a directory"; System.err.println(msg); } catch (IOException ioe) { String msg = args[0] + " could not be read, or " + args[1] + " could not be written"; System.err.println(msg); } finally { if (fos != null) try { fos.close(); } catch (IOException ioe) { System.err.println("unable to close " + args[1]); } } } catch (FileNotFoundException fnfe) { String msg = args[0] + " could not be found or might be a directory"; System.err.println(msg); } finally { if (fis != null) try CHAPTER 4: Mastering Advanced Language Features Part 1 174 { fis.close(); } catch (IOException ioe) { System.err.println("unable to close " + args[0]); } } } NOTE: Do not be concerned if you find this listing’s file-oriented code difficult to grasp; I will formally introduce I/O and the listing’s file-oriented types in Chapter 10. I’m presenting this code here because file copying provides a perfect example of the finally clause. Listing 4–30 presents an application that copies bytes from a source file to a destination file via a nested pair of try bodies. The outer try body uses a FileInputStream object to open the source file for reading; the inner try body uses a FileOutputStream object to create the destination file for writing, and also contains the file-copying code. If the fis = new FileInputStream(args[0]); expression throws FileNotFoundException, execution flows into the outer try statement’s catch (FileNotFoundException fnfe) clause, which outputs a suitable message to the user. Execution then enters the outer try statement’s finally clause. The outer try statement’s finally clause closes an open source file. However, when FileNotFoundException is thrown, the source file is not open—no reference was assigned to fis. The finally clause uses if (fis != null) to detect this situation, and does not attempt to close the file. If fis = new FileInputStream(args[0]); succeeds, execution flows into the inner try statement, whose body executes fos = new FileOutputStream(args[1]);. If this expression throws FileNotFoundException, execution moves into the inner try’s catch (FileNotFoundException fnfe) clause, which outputs a suitable message to the user. This time, execution continues with the inner try statement’s finally clause. Because the destination file was not created, no attempt is made to close this file. In contrast, the open source file must be closed, and this is accomplished when execution moves from the inner finally clause to the outer finally clause. FileInputStream’s and FileOutputStream’s close() methods throw IOException when a file is not open. Because IOException is checked, these exceptions must be handled; otherwise, it would be necessary to append a throws IOException clause to the main() method header. You can specify a try statement with only a finally clause. You would do so when you are not prepared to handle an exception in the enclosing method (or enclosing try statement, if present), but need to perform cleanup before the thrown exception causes execution to leave the method. Listing 4–31 provides a demonstration. This book was purchased by harshamv@variable3.comCHAPTER 4: Mastering Advanced Language Features Part 1 175 Listing 4–31. Cleaning up before handling a thrown exception public static void main(String[] args) { if (args.length != 2) { System.err.println("usage: java Copy srcfile dstfile"); return; } try { copy(args[0], args[1]); } catch (FileNotFoundException fnfe) { String msg = args[0] + " could not be found or might be a directory," + " or " + args[1] + " could not be created, " + "possibly because " + args[1] + " is a directory"; System.err.println(msg); } catch (IOException ioe) { String msg = args[0] + " could not be read, or " + args[1] + " could not be written"; System.err.println(msg); } } static void copy(String srcFile, String dstFile) throws IOException { FileInputStream fis = new FileInputStream(srcFile); try { FileOutputStream fos = new FileOutputStream(dstFile); try { int b; while ((b = fis.read()) != -1) fos.write(b); } finally { try { fos.close(); } catch (IOException ioe) { System.err.println("unable to close " + dstFile); } } } finally { try { fis.close(); } CHAPTER 4: Mastering Advanced Language Features Part 1 176 catch (IOException ioe) { System.err.println("unable to close " + srcFile); } } } Listing 4–31 provides an alternative to Listing 4–30 that attempts to be more readable. It accomplishes this task by introducing a copy() method that uses a nested pair of try- finally constructs to perform the file-copy operation, and also close each open file whether an exception is or is not thrown. If the FileInputStream fis = new FileInputStream(srcFile); expression results in a thrown FileNotFoundException, execution leaves copy() without entering the outer try statement. This statement is only entered after the FileInputStream object has been created, indicating that the source file was opened. If the FileOutputStream fos = new FileOutputStream(dstFile); expression results in a thrown FileNotFoundException, execution leaves copy() without entering the inner try statement. However, execution leaves copy() only after entering the finally clause that is mated with the outer try statement. This clause closes the open source file. If the read() or write() method in the inner try statement’s body throws an IOException object, the finally clause associated with the inner try statement is executed. This clause closes the open destination file. Execution then flows into the outer finally clause, which closes the open source file, and continues on out of copy(). CAUTION: If the body of a try statement throws an exception, and if the finally clause results in another exception being thrown, this new exception replaces the previous exception, which is lost. Despite Listing 4–31 being somewhat more readable than Listing 4–30, there is still too much boilerplate thanks to each finally clause requiring a try statement to close a file. This boilerplate is necessary; its removal results in a new IOException possibly being thrown from the catch clause, which would mask a previously thrown IOException. NOTE: Java version 7 introduces automatic resource management to eliminate the boilerplate associated with closing files and other resources. Furthermore, this feature can eliminate bugs that arise from masking thrown exceptions with other exceptions. Automatic resource management consists of a new Disposable interface that resource classes (such as FileInputStream) implement, and syntactic sugar that associates a semicolon- separated list of resource class instantiations with try. For example, automatic resource management can turn Listing 4–31’s copy() method into the following shorter method: CHAPTER 4: Mastering Advanced Language Features Part 1 177 static void copy(String srcFile, String dstFile) throws IOException { try (FileInputStream fis = new FileInputStream(srcFile); FileOutputStream fos = new FileOutputStream(dstFile)) { int b; while ((b = fis.read()) != -1) fos.write(b); } } EXERCISES The following exercises are designed to test your understanding of nested types, packages, static imports, and exceptions: 1. What is a nested class? 2. Identify the four kinds of nested classes. 3. Which nested classes are also known as inner classes? 4. True or false: A static member class has an enclosing instance. 5. How do you instantiate a nonstatic member class from beyond its enclosing class? 6. When is it necessary to declare local variables and parameters final? 7. True or false: An interface can be declared within a class or within another interface. 8. What is a package? 9. How do you ensure that package names are unique? 10. What is a package statement? 11. True or false: You can specify multiple package statements in a source file. 12. What is an import statement? 13. How do you indicate that you want to import multiple types via a single import statement? 14. During a runtime search, what happens when the virtual machine cannot find a classfile? 15. How do you specify the user classpath to the virtual machine? 16. What is a constant interface? 17. Why are constant interfaces used? 18. Why are constant interfaces bad? CHAPTER 4: Mastering Advanced Language Features Part 1 178 19. What is a static import statement? 20. How do you specify a static import statement? 21. What is an exception? 22. In what ways are objects superior to error codes for representing exceptions? 23. What is a throwable? 24. What does the getCause() method return? 25. What is the difference between Exception and Error? 26. What is a checked exception? 27. What is a runtime exception? 28. Under what circumstance would you introduce your own exception class? 29. True or false: You use a throw statement to identify exceptions that are thrown from a method by appending this statement to a method’s header. 30. What is the purpose of a try statement, and what is the purpose of a catch clause? 31. What is the purpose of a finally clause? 32. A 2D graphics package supports two-dimensional drawing and transformations (rotation, scaling, translation, and so on). These transformations require a 3-by-3 matrix (a table). Declare a G2D class that encloses a private Matrix nonstatic member class. Instantiate Matrix within G2D’s noargument constructor, and initialize the Matrix instance to the identity matrix (a matrix where all entries are 0 except for those on the upper-left to lower-right diagonal, which are 1). 33. Extend the logging package to support a null device in which messages are thrown away. 34. Modify the logging package so that Logger’s connect() method throws CannotConnectException when it cannot connect to its logging destination, and the other two methods each throw NotConnectedException when connect() was not called or when it threw CannotConnectException. 35. Modify TestLogger to respond appropriately to thrown CannotConnectException and NotConnectedException objects. Summary Classes that are declared outside of any class are known as top-level classes. Java also supports nested classes, which are classes declared as members of other classes or scopes. There are four kinds of nested classes: static member classes, nonstatic member classes, anonymous classes, and local classes. The latter three categories are known as inner classes. CHAPTER 4: Mastering Advanced Language Features Part 1 179 Java supports the partitioning of top-level types into multiple namespaces, to better organize these types and to also prevent name conflicts. Java uses packages to accomplish these tasks. The package statement identifies the package in which a source file’s types are located. The import statement imports types from a package by telling the compiler where to look for unqualified type names during compilation. An exception is a divergence from an application’s normal behavior. Although it can be represented by an error code or object, Java uses objects because error codes are meaningless and cannot contain information about what led to the exception. Java provides a hierarchy of classes that represent different kinds of exceptions. These classes are rooted in Throwable. Moving down the throwable hierarchy, you encounter the Exception and Error classes, which represent nonerror exceptions and errors. Exception and its subclasses, except for RuntimeException (and its subclasses), describe checked exceptions. They are checked because you must check the code to ensure that an exception is handled where thrown or identified as being handled elsewhere. RuntimeException and its subclasses describe unchecked exceptions. You do not have to handle these exceptions because they represent coding mistakes (fix the mistakes). Although the names of their classes can appear in throws clauses, doing so adds clutter. The throw statement throws an exception to the virtual machine, which searches for an appropriate handler. If the exception is checked, its name must appear in the method’s throws clause, unless the name of the exception’s superclass is listed in this clause. A method handles one or more exceptions by specifying a try statement and appropriate catch clauses. A finally clause can be included to execute cleanup code whether an exception is thrown or not, and before a thrown exception leaves the method. Now that you have mastered the advanced language features related to nested types, packages, static imports, and exceptions, you can leverage this knowledge in Chapter 5, where you explore features related to assertions, annotations, generics, and enums. CHAPTER 4: Mastering Advanced Language Features Part 1 180 181 181 Chapter Mastering Advanced Language Features Part 2 Chapters 2 and 3 laid a foundation for learning the Java language, and Chapter 4 built onto this foundation by introducing some of Java’s more advanced language features. Chapter 5 continues to cover advanced language features by focusing on those features related to assertions, annotations, generics, and enums. Assertions Writing source code is not an easy task. All too often, bugs (defects) are introduced into the code. When a bug is not discovered before compiling the source code, it makes it into runtime code, which will probably fail unexpectedly. At this point, the cause of failure can be very difficult to determine. Developers often make assumptions about application correctness, and some developers think that specifying comments that state their beliefs about what they think is true at the comment locations is sufficient for determining correctness. However, comments are useless for preventing bugs because the compiler ignores them. Many languages address this problem by providing an assertions language feature that lets the developer codify assumptions about application correctness. When the application runs, if an assertion fails, the application terminates with a message that helps the developer diagnose the failure’s cause. This section introduces you to Java’s assertions language feature. After defining this term, showing you how to declare assertions, and providing examples, the section looks at using and avoiding assertions. Finally, you learn how to selectively enable and disable assertions via the Java SE 6u16 javac compiler tool’s command-line arguments. 5 CHAPTER 5: Mastering Advanced Language Features Part 2 182 Declaring Assertions An assertion is a statement that lets you express an assumption of program correctness via a Boolean expression. If this expression evaluates to true, execution continues with the next statement. Otherwise, an error that identifies the cause of failure is thrown. There are two forms of the assertion statement, with each form beginning with reserved word assert: assert expression1 ; assert expression1 : expression2 ; In both forms of this statement, expression1 is the Boolean expression. In the second form, expression2 is any expression that returns a value. It cannot be a call to a method whose return type is void. When expression1 evaluates to false, this statement instantiates the AssertionError class. The first statement form calls this class’s noargument constructor, which does not associate a message identifying failure details with the AssertionError instance. The second form calls an AssertionError constructor whose type matches the type of expression2’s value. This value is passed to the constructor and its string representation is used as the error’s detail message. When the error is thrown, the name of the source file and the number of the line from where the error was thrown are output to the console as part of the thrown error’s stack trace. In many situations, this information is sufficient for identifying what led to the failure, and the first form of the assertion statement should be used. Listing 5–1 demonstrates the first form of the assertion statement. Listing 5–1. Throwing an assertion error without a detail message public class AssertionDemo { public static void main(String[] args) { int x = 1; assert x == 0; } } When assertions are enabled (I discuss this task later), running the previous application results in the following output: Exception in thread "main" java.lang.AssertionError at AssertionDemo.main(AssertionDemo.java:6) In other situations, more information is needed to help diagnose the cause of failure. For example, suppose expression1 compares variables x and y, and throws an error when x’s value exceeds y’s value. Because this should never happen, you would probably use the second statement form to output these values so you could diagnose the problem. Listing 5–2 demonstrates the second form of the assertion statement. CHAPTER 5: Mastering Advanced Language Features Part 2 183 Listing 5–2. Throwing an assertion error with a detail message public class AssertionDemo { public static void main(String[] args) { int x = 1; assert x == 0: x; } } Once again, it is assumed that assertions are enabled. Running the previous application results in the following output: Exception in thread "main" java.lang.AssertionError: 1 at AssertionDemo.main(AssertionDemo.java:6) The value in x is appended to the end of the first output line, which is somewhat cryptic. To make this output more meaningful, you might want to specify an expression that also includes the variable’s name: assert x == 0: "x = " + x;, for example. Using Assertions There are many situations where assertions should be used. These situations organize into internal invariant, control-flow invariant, and design-by-contract categories. An invariant is something that does not change. Internal Invariants An internal invariant is expression-oriented behavior that is not expected to change. For example, Listing 5–3 introduces an internal invariant by way of chained if-else statements that output the state of water based on its temperature. Listing 5–3. Discovering that an internal invariant can vary public class IIDemo { public static void main(String[] args) { double temperature = 50.0; // Celsius if (temperature < 0.0) System.out.println("water has solidified"); else if (temperature >= 100.0) System.out.println("water is boiling into a gas"); else { // temperature > 0.0 and temperature < 100.0 assert(temperature > 0.0 && temperature < 100.0): temperature; System.out.println("water is remaining in its liquid state"); } } } CHAPTER 5: Mastering Advanced Language Features Part 2 184 A developer might specify only a comment stating an assumption as to what expression causes the final else to be reached. Because the comment might not be enough to detect the lurking < 0.0 expression bug, an assertion statement is necessary. Another example of an internal invariant concerns a switch statement with no default case. The default case is avoided because the developer believes that all paths have been covered. However, this is not always true, as Listing 5–4 demonstrates. Listing 5–4. Another buggy internal invariant public class IIDemo { final static int NORTH = 0; final static int SOUTH = 1; final static int EAST = 2; final static int WEST = 3; public static void main(String[] args) { int direction = (int)(Math.random()*5); switch (direction) { case NORTH: System.out.println("travelling north"); break; case SOUTH: System.out.println("travelling south"); break; case EAST : System.out.println("travelling east"); break; case WEST : System.out.println("travelling west"); break; default : assert false; } } } Listing 5–4 assumes that the expression tested by switch will only evaluate to one of four integer constants. However, (int)(Math.random()*5) can also return 4, causing the default case to execute assert false;, which always throws AssertionError. (You might have to run this application a few times to see the assertion error, but first you need to learn how to enable assertions, which I discuss later in this chapter.) TIP: When assertions are disabled, assert false; does not execute and the bug goes undetected. To always detect this bug, replace assert false; with throw new AssertionError(direction);. Control-Flow Invariants A control-flow invariant is a flow of control that is not expected to change. For example, Listing 5–4 uses an assertion to test an assumption that switch’s default case will not execute. Listing 5–5, which fixes Listing 5–4’s bug, provides another example. Listing 5–5. A buggy control-flow invariant public class CFDemo { final static int NORTH = 0; final static int SOUTH = 1; This book was purchased by harshamv@variable3.comCHAPTER 5: Mastering Advanced Language Features Part 2 185 final static int EAST = 2; final static int WEST = 3; public static void main(String[] args) { int direction = (int)(Math.random()*4); switch (direction) { case NORTH: System.out.println("travelling north"); break; case SOUTH: System.out.println("travelling south"); break; case EAST : System.out.println("travelling east"); break; case WEST : System.out.println("travelling west"); default : assert false; } } } Because the original bug has been fixed, the default case should never be reached. However, the omission of a break statement that terminates case WEST causes execution to reach the default case. This control-flow invariant has been broken. (Again, you might have to run this application a few times to see the assertion error, but first you need to learn how to enable assertions, which I discuss later in this chapter.) CAUTION: Be careful when using an assertion statement to detect code that should never be executed. If the assertion statement cannot be reached according to the rules set forth in The Java Language Specification, Third Edition, by James Gosling, Bill Joy, Guy Steele, and Gilad Bracha (Addison-Wesley, 2005; ISBN: 0321246780) (also available at http://java.sun.com/docs/books/jls/third_edition/html/j3TOC.html), the compiler will report an error. For example, for(;;); assert false; causes the compiler to report an error because the infinite for loop prevents the assertion statement from executing. Design-by-Contract Design-by-Contract is a way to design software based on preconditions, postconditions, and invariants (internal, control-flow, and class). Assertion statements support an informal design-by-contract style of development. Preconditions A precondition is something that must be true when a method is called. Assertion statements are often used to satisfy a helper method’s preconditions by checking that its arguments are legal. Listing 5–6 provides an example. Listing 5–6. Verifying a precondition public class Lotto649 { public static void main(String[] args) { // Lotto 649 requires that six unique numbers be chosen. CHAPTER 5: Mastering Advanced Language Features Part 2 186 int[] selectedNumbers = new int[6]; // Assign a unique random number from 1 to 49 (inclusive) to each slot // in the selectedNumbers array. for (int slot = 0; slot < selectedNumbers.length; slot++) { int num; // Obtain a random number from 1 to 49. That number becomes the // selected number if it has not previously been chosen. try_again: do { num = rnd(49)+1; for (int i = 0; i < slot; i++) if (selectedNumbers[i] == num) continue try_again; break; } while (true); // Assign selected number to appropriate slot. selectedNumbers[slot] = num; } // Sort all selected numbers into ascending order and then print these // numbers. sort(selectedNumbers); for (int i = 0; i < selectedNumbers.length; i++) System.out.print(selectedNumbers[i] + " "); } private static int rnd(int limit) { // This method returns a random number (actually, a pseudorandom number) // ranging from 0 through limit-1 (inclusive). assert limit > 1: "limit = " + limit; return (int)(Math.random()*limit); } private static void sort(int[] x) { // This method sorts the integers in the passed array into ascending // order. for (int pass = 0; pass < x.length-1; pass++) for (int i = x.length-1; i > pass; i--) if (x[i] < x[pass]) { int temp = x[i]; x[i] = x[pass]; x[pass] = temp; } } } Listing 5–6’s application simulates Lotto 6/49, one of Canada’s national lottery games. The rnd() helper method returns a randomly chosen integer between 0 and limit-1. An assertion statement verifies the precondition that limit’s value must be 2 or higher. CHAPTER 5: Mastering Advanced Language Features Part 2 187 NOTE: The sort() helper method sorts (orders) the selectedNumbers array’s integers into ascending order by implementing an algorithm (a recipe for accomplishing some task) called Bubble Sort. Bubble Sort works by making multiple passes over the array. During each pass, various comparisons and swaps ensure that the next smallest element value “bubbles” toward the top of the array, which would be the element at index 0. Bubble Sort is not efficient, but is more than adequate for sorting a six-element array. Although I could have used one of the efficient sort() methods located in the java.util package’s Arrays class (for example, Arrays.sort(selectedNumbers); accomplishes the same objective as Listing 5–6’s sort(selectedNumbers); method call, but does so more efficiently), I chose to use Bubble Sort because I prefer to wait until Chapter 8 before getting into the Arrays class. Postconditions A postcondition is something that must be true after a method successfully completes. Assertion statements are often used to satisfy a helper method’s postconditions by checking that its result is legal. Listing 5–7 provides an example. Listing 5–7. Verifying a postcondition in addition to preconditions public class MergeArrays { public static void main(String[] args) { int[] x = { 1, 2, 3, 4, 5 }; int[] y = { 1, 2, 7, 9 }; int[] result = merge(x, y); for (int i = 0; i < result.length; i++) System.out.println(result[i]); } public static int[] merge(int[] a, int[] b) { if (a == null) throw new NullPointerException("a is null"); if (b == null) throw new NullPointerException("b is null"); int[] result = new int[a.length+b.length]; // Precondition assert result.length == a.length+b.length: "length mismatch"; for (int i = 0; i < a.length; i++) result[i] = a[i]; for (int i = 0; i < b.length; i++) result[a.length+i-1] = b[i]; // Postcondition assert containsAll(result, a, b): "value missing from array"; return result; } CHAPTER 5: Mastering Advanced Language Features Part 2 188 private static boolean containsAll(int[] result, int[] a, int[] b) { for (int i = 0; i < a.length; i++) if (!contains(result, a[i])) return false; for (int i = 0; i < b.length; i++) if (!contains(result, b[i])) return false; return true; } private static boolean contains(int[] a, int val) { for (int i = 0; i < a.length; i++) if (a[i] == val) return true; return false; } } Listing 5–7 uses an assertion statement to verify the postcondition that all of the values in the two arrays being merged are present in the merged array. The postcondition is not satisfied, however, because this listing contains a bug. Listing 5–7 also shows preconditions and postconditions being used together. The solitary precondition verifies that the merged array length equals the lengths of the arrays being merged prior to the merge logic. Class Invariants A class invariant is a kind of internal invariant that applies to every instance of a class at all times, except when an instance is transitioning from one consistent state to another. For example, suppose instances of a class contain arrays whose values are sorted in ascending order. You might want to include an isSorted() method in the class that returns true if the array is still sorted, and verify that each constructor and method that modifies the array specifies assert isSorted(); prior to exit, to satisfy the assumption that the array is still sorted when the constructor/method exists. Avoiding Assertions Although there are many situations where assertions should be used, there also are situations where they should be avoided. For example, you should not use assertions to check the arguments that are passed to public methods, for the following reasons:  Checking a public method’s arguments is part of the contract that exists between the method and its caller. If you use assertions to check these arguments, and if assertions are disabled, this contract is violated because the arguments will not be checked. CHAPTER 5: Mastering Advanced Language Features Part 2 189  Assertions also prevent appropriate exceptions from being thrown. For example, when an illegal argument is passed to a public method, it is common to throw IllegalArgumentException or NullPointerException. However, AssertionError is thrown instead. You should also avoid using assertions to perform work required by the application to function correctly. This work is often performed as a side effect of the assertion’s Boolean expression. When assertions are disabled, the work is not performed. For example, suppose you have a list of Employee objects and a few null references that are also stored in this list, and you want to remove all of the null references. It would not be correct to remove these references via the following assertion statement: assert employees.removeAll(null); Although the assertion statement will not throw AssertionError because there is at least one null reference in the employees list, the application that depends upon this statement executing will fail when assertions are disabled. Instead of depending on the former code to remove the null references, you would be better off using code similar to the following: boolean allNullsRemoved = employees.removeAll(null); assert allNullsRemoved; This time, all null references are removed regardless of whether assertions are enabled or disabled, and you can still specify an assertion to verify that nulls were removed. Enabling and Disabling Assertions The compiler records assertions in the classfile. However, assertions are disabled at runtime because they can affect performance. An assertion might call a method that takes awhile to complete, and this would impact the running application’s performance. You must enable the classfile’s assertions before you can test assumptions about the behaviors of your classes. Accomplish this task by specifying the -enableassertions or -ea command-line option when running the java application launcher tool. The -enableassertions and -ea command-line options let you enable assertions at various granularities based upon one of the following arguments (except for the noargument scenario, you must use a colon to separate the option from its argument):  No argument: Assertions are enabled in all classes except system classes.  PackageName...: Assertions are enabled in the specified package and its subpackages by specifying the package name followed by ....  ...: Assertions are enabled in the unnamed package, which happens to be whatever directory is current.  ClassName: Assertions are enabled in the named class by specifying the class name. CHAPTER 5: Mastering Advanced Language Features Part 2 190 For example, you can enable all assertions except system assertions when running the MergeArrays application via java –ea MergeArrays. Also, you could enable any assertions in Chapter 4’s logging package by specifying java –ea:logging TestLogger. Assertions can be disabled, and also at various granularities, by specifying either of the –disableassertions or –da command-line options. These options take the same arguments as -enableassertions and -ea. For example, java -ea –da:loneclass mainclass enables all assertions except for those in loneclass. (loneclass and mainclass are placeholders for the actual classes that you specify.) The previous options apply to all classloaders. Except when taking no arguments, they also apply to system classes. This exception simplifies the enabling of assertion statements in all classes except for system classes, which is often desirable. To enable system assertions, specify either -enablesystemassertions or -esa; for example, java -esa –ea:logging TestLogger. Specify either -disablesystemassertions or -dsa to disable system assertions. Annotations While developing a Java application, you might want to annotate, or associate metadata (data that describes other data) with, various application elements. For example, you might want to identify methods that are not fully implemented so that you will not forget to implement them. Java’s annotations language feature lets you accomplish this task. This section introduces you to annotations. After defining this term and presenting three kinds of compiler-supported annotations as examples, the section shows you how to declare your own annotation types and use these types to annotate source code. Finally, you discover how to process your own annotations to accomplish useful tasks. NOTE: Java has always supported ad hoc annotation mechanisms. For example, the java.lang.Cloneable interface identifies classes whose instances can be shallowly cloned via Object’s clone() method, the transient reserved word marks fields that are to be ignored during serialization, and the @deprecated javadoc tag documents methods that are no longer supported. In contrast, the annotations feature is a standard for annotating code. Discovering Annotations An annotation is an instance of an annotation type and associates metadata with an application element. It is expressed in source code by prefixing the type name with the @ symbol. For example, @Readonly is an annotation and Readonly is its type. CHAPTER 5: Mastering Advanced Language Features Part 2 191 NOTE: You can use annotations to associate metadata with constructors, fields, local variables, methods, packages, parameters, and types (annotation, class, enum, and interface). Beginning with Java version 7, annotations can appear on any use of a type. For example, you might declare Employee[@Readonly] emps; to indicate that emps is an unmodifiable one- dimensional array of mutable (modifiable) Employee instances. The compiler supports the Override, Deprecated, and SuppressWarnings annotation types. These types are located in the java.lang package. @Override annotations are useful for expressing that a subclass method overrides a method in the superclass, and does not overload that method instead. Listing 5–8 reveals that this annotation prefixes the overriding method. Listing 5–8. Annotating an overriding method @Override public void draw(int color) { // drawing code } NOTE: In Chapter 3, I presented @Override on the same line as the method header. In Listing 5–8, I present this annotation on a separate line above the method header. It is common practice to place annotations on separate lines above the application elements that they annotate, unless they are being used to annotate parameters. In that case, an annotation must appear on the same line as and in front of the parameter’s name. @Deprecated annotations are useful for indicating that the marked application element is deprecated (phased out) and should no longer be used. The compiler warns you when a deprecated application element is accessed by nondeprecated code. In contrast, the @deprecated javadoc tag and associated text warns you against using the deprecated item, and tells you what to use instead. Listing 5–9 demonstrates that @Deprecated and @deprecated can be used together. Listing 5–9. Deprecating a method via @Deprecated and @deprecated /** * Allocates a Date object and initializes it so that * it represents midnight, local time, at the beginning of the day * specified by the year, month, and * date arguments. * * @param year the year minus 1900. * @param month the month between 0-11. * @param date the day of the month between 1-31. * @see java.util.Calendar * @deprecated As of JDK version 1.1, CHAPTER 5: Mastering Advanced Language Features Part 2 192 * replaced by Calendar.set(year + 1900, month, date) * or GregorianCalendar(year + 1900, month, date). */ @Deprecated public Date(int year, int month, int date) { this(year, month, date, 0, 0, 0); } Listing 5–9 excerpts one of the constructors in Java’s Date class (located in the java.util package). This constructor has been deprecated in favor of using the set() method in the Calendar class (also located in the java.util package). The compiler suppresses warnings if a compilation unit (typically a class or interface) refers to a deprecated class, method, or field. This feature lets you modify legacy APIs without generating deprecation warnings, and is demonstrated in Listing 5–10. Listing 5–10. Referencing a deprecated field from within the same class declaration public class Employee { /** * Employee's name * @deprecated New version uses firstName and lastName fields. */ @Deprecated String name; String firstName; String lastName; public static void main(String[] args) { Employee emp = new Employee(); emp.name = "John Doe"; } } Listing 5–10 declares an Employee class with a name field that has been deprecated. Although Employee’s main() method refers to name, the compiler will suppress a deprecation warning because the deprecation and reference occur in the same class. Suppose you refactor Listing 5–10 by introducing a new UseEmployee class and moving Employee’s main() method to this class. Listing 5–11 presents the resulting class structure. Listing 5–11. Referencing a deprecated field from within another class declaration class Employee { /** * Employee's name * @deprecated New version uses firstName and lastName fields. */ @Deprecated String name; String firstName; String lastName; } CHAPTER 5: Mastering Advanced Language Features Part 2 193 public class UseEmployee { public static void main(String[] args) { Employee emp = new Employee(); emp.name = "John Doe"; } } If you attempt to compile this source code via Java SE 6u16’s javac compiler tool, you will discover the following messages: Note: Employee.java uses or overrides a deprecated API. Note: Recompile with -Xlint:deprecation for details. You will need to specify -Xlint:deprecation as one of javac’s command-line arguments to discover the deprecated item and the code that refers to this item: Employee.java:17: warning: [deprecation] name in Employee has been deprecated emp.name = "John Doe"; ^ 1 warning @SuppressWarnings annotations are useful for suppressing deprecation or unchecked warnings via a "deprecation" or an "unchecked" argument. (Unchecked warnings occur when mixing code that uses generics with pre-generics legacy code.) For example, Listing 5–12 uses @SuppressWarnings with a "deprecation" argument to suppress the compiler’s deprecation warnings when code within the UseEmployee class’s main() method accesses the Employee class’s name field. Listing 5–12. Suppressing the previous deprecation warning public class UseEmployee { @SuppressWarnings("deprecation") public static void main(String[] args) { Employee emp = new Employee(); emp.name = "John Doe"; } } Declaring Annotation Types and Annotating Source Code Before you can annotate source code, you need annotation types that can be instantiated. Java supplies many annotation types in addition to Override, Deprecated, and SuppressWarnings. Java also lets you declare your own types. You declare an annotation type by specifying the @ symbol, immediately followed by reserved word interface, followed by the type’s name, followed by a body. For example, Listing 5–13 uses @interface to declare an annotation type named Stub. Listing 5–13. Declaring the Stub annotation type public @interface Stub CHAPTER 5: Mastering Advanced Language Features Part 2 194 { } Instances of annotation types that supply no data apart from a name—their bodies are empty—are known as marker annotations because they mark application elements for some purpose. As Listing 5–14 reveals, @Stub is used to mark empty methods (stubs). Listing 5–14. Annotating a stubbed-out method public class Deck // Describes a deck of cards. { @Stub public void shuffle() { // This method is empty and will presumably be filled in with appropriate // code at some later date. } } Listing 5–14’s Deck class declares an empty shuffle() method. This fact is indicated by instantiating Stub and prefixing shuffle()’s method header with the resulting @Stub annotation. NOTE: Although marker interfaces (discussed in Chapter 3) appear to have been replaced by marker annotations, this is not the case, because marker interfaces have advantages over marker annotations. One advantage is that a marker interface specifies a type that is implemented by a marked class, which lets you catch problems at compile time. For example, if a class does not implement the Cloneable interface, its instances cannot be shallowly cloned via Object’s clone() method. If Cloneable had been implemented as a marker annotation, this problem would not be detected until runtime. Although marker annotations are useful (@Override and @Deprecated are good examples), you will typically want to enhance an annotation type so that you can store metadata via its instances. You accomplish this task by adding elements to the type. An element is a method header that appears in the annotation type’s body. It cannot have parameters or a throws clause, and its return type must be a primitive type (such as int), String, Class, an enum, an annotation type, or an array of the preceding types. However, it can have a default value. Listing 5–15 adds three elements to Stub. Listing 5–15. Adding three elements to the Stub annotation type public @interface Stub { int id(); // A semicolon must terminate an element declaration. String dueDate(); String developer() default "unassigned"; } This book was purchased by harshamv@variable3.comCHAPTER 5: Mastering Advanced Language Features Part 2 195 The id() element specifies a 32-bit integer that identifies the stub. The dueDate() element specifies a String-based date that identifies when the method stub is to be implemented. Finally, developer() specifies the String-based name of the developer responsible for coding the method stub. Unlike id() and dueDate(), developer() is declared with a default value, "unassigned". When you instantiate Stub and do not assign a value to developer() in that instance, as is the case with Listing 5–16, this default value is assigned to developer(). Listing 5–16. Initializing a Stub instance’s elements public class Deck { @Stub ( id = 1, dueDate = "10/21/2010" ) public void shuffle() { } } Listing 5–16 reveals one @Stub annotation that initializes its id() element to 1 and its dueDate() element to "10/21/2010". Each element name does not have a trailing (), and the comma-separated list of two element initializers appears between ( and ). Suppose you decide to replace Stub’s id(), dueDate(), and developer() elements with a single String value() element whose string specifies comma-separated ID, due date, and developer name values. Listing 5–17 shows you two ways to initialize value. Listing 5–17. Initializing each Stub instance’s value() element public class Deck { @Stub(value = "1,10/21/2010,unassigned") public void shuffle() { } @Stub("2,10/21/2010,unassigned") public Card[] deal(int ncards) { return null; } } This listing reveals special treatment for the value() element. When it is an annotation type’s only element, you can omit value()’s name and = from the initializer. I used this fact to specify @SuppressWarnings("deprecation") in Listing 5–12. CHAPTER 5: Mastering Advanced Language Features Part 2 196 Using Meta-Annotations in Annotation Type Declarations Each of the Override, Deprecated, and SuppressWarnings annotation types is itself annotated with meta-annotations (annotations that annotate annotation types). For example, Listing 5–18 shows you that the SuppressWarnings annotation type is annotated with two meta-annotations. Listing 5–18. The annotated SuppressWarnings type declaration @Target(value={TYPE,FIELD,METHOD,PARAMETER,CONSTRUCTOR,LOCAL_VARIABLE}) @Retention(value=SOURCE) public @interface SuppressWarnings The Target annotation type, which is located in the java.lang.annotation package, identifies the kinds of application elements to which an annotation type applies. @Target indicates that @SuppressWarnings annotations can be used to annotate types, fields, methods, parameters, constructors, and local variables. Each of TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, and LOCAL_VARIABLE is a member of the ElementType enum, which is also located in the java.lang.annotation package. The { and } characters surrounding the comma-separated list of values assigned to Target’s value() element signify an array—value()’s return type is String[]. Although these braces are necessary (unless the array consists of one item), value= could be omitted when initializing @Target because Target declares only a value() element. The Retention annotation type, which is located in the java.lang.annotation package, identifies the retention (also known as lifetime) of an annotation type’s annotations. @Retention indicates that @SuppressWarnings annotations have a lifetime that is limited to source code—they do not exist after compilation. SOURCE is one of the members of the RetentionPolicy enum (located in the java.lang.annotation package). The other members are CLASS and RUNTIME. These three members specify the following retention policies:  CLASS: The compiler records annotations in the classfile, but the virtual machine does not retain them (to save memory space). This policy is the default.  RUNTIME: The compiler records annotations in the classfile, and the virtual machine retains them so that they can be read via the Reflection API (discussed in Chapter 7) at runtime.  SOURCE: The compiler discards annotations after using them. There are two problems with the Stub annotation type shown in Listings 5–13 and 5–15. First, the lack of an @Target meta-annotation means that you can annotate any application element @Stub. However, this annotation only makes sense when applied to methods and constructors. Check out Listing 5–19. CHAPTER 5: Mastering Advanced Language Features Part 2 197 Listing 5–19. Annotating undesirable application elements @Stub("1,10/21/2010,unassigned") public class Deck { @Stub("2,10/21/2010,unassigned") private Card[] cardsRemaining; @Stub("3,10/21/2010,unassigned") public Deck() { } @Stub("4,10/21/2010,unassigned") public void shuffle() { } @Stub("5,10/21/2010,unassigned") public Card[] deal(@Stub("5,10/21/2010,unassigned") int ncards) { return null; } } Listing 5–19 uses @Stub to annotate the Deck class, the cardsRemaining field, and the ncards parameter in addition to annotating the constructor and the two methods. The first three application elements are inappropriate to annotate because they are not stubs. You can fix this problem by prefixing the Stub annotation type declaration with @Target({ElementType.METHOD, ElementType.CONSTRUCTOR}) so that Stub only applies to methods and constructors. After doing this, the Java SE 6u16 javac compiler tool will output the following error messages when you attempt to compile Listing 5–19: Deck.java:1: annotation type not applicable to this kind of declaration @Stub("1,10/21/2010,unassigned") ^ Deck.java:4: annotation type not applicable to this kind of declaration @Stub("2,10/21/2010,unassigned") ^ Deck.java:15: annotation type not applicable to this kind of declaration public Card[] deal(@Stub("5,10/21/2010,unassigned") int ncards) ^ 3 errors The second problem is that the default CLASS retention policy makes it impossible to process @Stub annotations at runtime. You can fix this problem by prefixing the Stub type declaration with @Retention(RetentionPolicy.RUNTIME). Listing 5–20 presents the Stub annotation type with the desired @Target and @Retention meta-annotations. Listing 5–20. A revamped Stub annotation type @Target({ElementType.METHOD, ElementType.CONSTRUCTOR}) @Retention(RetentionPolicy.RUNTIME) public @interface Stub { String value(); } CHAPTER 5: Mastering Advanced Language Features Part 2 198 NOTE: Java also provides Documented and Inherited meta-annotation types in the java.lang.annotation package. Instances of @Documented-annotated annotation types are to be documented by javadoc and similar tools. Instances of @Inherited-annotated annotation types are automatically inherited. According to Inherited’s Java documentation, if “the user queries the annotation type on a class declaration, and the class declaration has no annotation for this type, then the class’s superclass will automatically be queried for the annotation type. This process will be repeated until an annotation for this type is found, or the top of the class hierarchy (Object) is reached. If no superclass has an annotation for this type, then the query will indicate that the class in question has no such annotation.” Processing Annotations It is not enough to declare an annotation type and use that type to annotate source code. Unless you do something specific with those annotations, they remain dormant. One way to accomplish something specific is to write an application that processes the annotations. Listing 5–21’s StubFinder application does just that. Listing 5–21. The StubFinder application import java.lang.reflect.*; public class StubFinder { public static void main(String[] args) throws Exception { if (args.length != 1) { System.err.println("usage: java StubFinder classfile"); return; } Method[] methods = Class.forName(args[0]).getMethods(); for (int i = 0; i < methods.length; i++) if (methods[i].isAnnotationPresent(Stub.class)) { Stub stub = methods[i].getAnnotation(Stub.class); String[] components = stub.value().split(","); System.out.println("Stub ID = " + components[0]); System.out.println("Stub Date = " + components[1]); System.out.println("Stub Developer = " + components[2]); System.out.println(); } } } StubFinder loads a classfile whose name is specified as a command-line argument, and outputs the metadata associated with each @Stub annotation that precedes each public method header. These annotations are instances of Listing 5–20’s Stub annotation type. CHAPTER 5: Mastering Advanced Language Features Part 2 199 StubFinder next uses a special class named Class and its forName() class method to load a classfile. Class also provides a getMethods() method that returns an array of Method objects describing the loaded class’s public methods. For each loop iteration, a Method object’s isAnnotationPresent() method is called to determine if the method is annotated with the annotation described by the Stub class (referred to as Stub.class). If isAnnotationPresent() returns true, Method’s getAnnotation() method is called to return the annotation Stub instance. This instance’s value() method is called to retrieve the string stored in the annotation. Next, String’s split() method is called to split the string’s comma-separated list of ID, date, and developer values into an array of String objects. Each object is then output along with descriptive text. Class’s forName() method is capable of throwing various exceptions that must be handled or explicitly declared as part of a method’s header. For simplicity, I chose to append a throws Exception clause to the main() method’s header. CAUTION: There are two problems with throws Exception. First, it is better to handle the exception and present a suitable error message than to “pass the buck” by throwing it out of main(). Second, Exception is generic—it hides the names of the kinds of exceptions that are thrown. However, it is convenient to specify throws Exception in a throwaway utility. Do not be concerned if you do not understand Class, forName(), getMethods(), Method, isAnnotationPresent(), .class, getAnnotation(), and split(). You will learn about these items in Chapter 7. After compiling StubFinder (javac StubFinder.java), Stub (javac Stub.java), and Listing 5–17’s Deck class (javac Deck.java), run StubFinder with Deck as its single command-line argument (java StubFinder Deck). You will observe the following output: Stub ID = 2 Stub Date = 10/21/2010 Stub Developer = unassigned Stub ID = 1 Stub Date = 10/21/2010 Stub Developer = unassigned If you expected the output to reflect the order of appearance of @Stub annotations in Deck.java, you are probably surprised by the output’s unsorted order. This lack of order is caused by getMethods(). According to this method’s Java documentation, “the elements in the array returned are not sorted and are not in any particular order.” CHAPTER 5: Mastering Advanced Language Features Part 2 200 NOTE: Java version 5 introduced an apt tool for processing annotations. This tool’s functionality has been integrated into the compiler beginning with Java version 6—apt is being phased out. My “Processing Annotations in Java SE 6” article (http://javajeff.mb.ca/cgi- bin/mp.cgi?a=/java/javase/articles/paijse6) provides a tutorial on using the Java version 6 compiler to process annotations. Generics Java version 5 introduced generics, language features for declaring and using type- agnostic classes and interfaces. When working with Java’s collections framework (which I introduce in Chapter 8), these features help you avoid java.lang.ClassCastExceptions. NOTE: Although the main use for generics is the collections framework, Java’s class library also contains generified (retrofitted to make use of generics) classes that have nothing to do with this framework: java.lang.Class, java.lang.ThreadLocal, and java.lang.ref.WeakReference are three examples. This section introduces you to generics. You first learn how generics promote type safety in the context of the collections classes, and then you explore generics in the contexts of generic types and generic methods. Collections and the Need for Type Safety Java’s collections framework makes it possible to store objects in various kinds of containers (known as collections) and later retrieve those objects. For example, you can store objects in a list, a set, or a map. You can then retrieve a single object, or iterate over the collection and retrieve all objects. Before Java version 5 overhauled the collections framework to take advantage of generics, there was no way to prevent a collection from containing objects of mixed types. The compiler did not check an object’s type to see if it was suitable before it was added to a collection, and this lack of static type checking led to ClassCastExceptions. Listing 5–22 demonstrates how easy it is to generate a ClassCastException. Listing 5–22. Lack of type safety leading to a ClassCastException at runtime public static void main(String[] args) { List employees = new ArrayList(); employees.add(new Employee("John Doe")); employees.add(new Employee("Jane Doe")); employees.add("Doe Doe"); Iterator iter = employees.iterator(); while (iter.hasNext()) CHAPTER 5: Mastering Advanced Language Features Part 2 201 { Employee emp = (Employee) iter.next(); System.out.println(emp.getName()); } } After instantiating ArrayList, main() uses this list collection object’s reference to add a pair of Employee objects to the list. It then adds a String object, which violates the implied contract that ArrayList is supposed to store only Employee objects. Moving on, main() obtains an Iterator for iterating over the list of Employees. As long as Iterator’s hasNext() method returns true, its next() method is called to return an object stored in the ArrayList. The Object that next() returns must be downcast to Employee so that the Employee object’s getName() method can be called to return the employee’s name. The string that this method returns is then output to the standard output device via System.out.println(). The (Employee) cast checks the type of each object returned by next() to make sure that it is an Employee. Although this is true of the first two objects, it is not true of the third object. Attempting to cast "Doe Doe" to Employee results in a ClassCastException. The ClassCastException occurs because of an assumption that a list is homogenous. In other words, a list stores only objects of a single type or a family of related types. In reality, the list is heterogeneous in that it can store any Object. Listing 5–23’s generics-based homogenous list avoids ClassCastException. Listing 5–23. Lack of type safety leading to a compiler error public static void main(String[] args) { List employees = new ArrayList(); employees.add(new Employee("John Doe")); employees.add(new Employee("Jane Doe")); employees.add("Doe Doe"); Iterator iter = employees.iterator(); while (iter.hasNext()) { Employee emp = iter.next(); System.out.println(emp.getName()); } } Three of Listing 5–23’s bolded code fragments illustrate the central feature of generics, which is the parameterized type (a class or interface name followed by an angle bracket– delimited type list identifying what kinds of objects are legal in that context). For example, List indicates only Employee objects can be stored in the List. The designation must be repeated with ArrayList, which is the collection implementation that stores the Employees. CHAPTER 5: Mastering Advanced Language Features Part 2 202 NOTE: Java version 7 introduces the diamond operator (<>) to save you from having to repeat a parameterized type. For example, you could use this operator to specify List employees = new ArrayList<>();. Also, Iterator indicates that iterator() returns an Iterator whose next() method returns only Employee objects. It is not necessary to cast iter.next()’s returned value to Employee because the compiler inserts the cast on your behalf. If you attempt to compile this listing, the compiler will report an error when it encounters employees.add("Doe Doe");. The error message will tell you that the compiler cannot find an add(java.lang.String) method in the java.util.List interface. Unlike in the pre-generics List interface, which declares an add(Object) method, the generified List interface’s add() method parameter reflects the interface’s parameterized type name. For example, List implies add(Employee). Listing 5–22 revealed that the unsafe code causing the ClassCastException (employees.add("Doe Doe");) and the code that triggers the exception ((Employee) iter.next()) are quite close. However, they are often farther apart in larger applications. Rather than having to deal with angry clients while hunting down the unsafe code that ultimately led to the ClassCastException, you can rely on the compiler saving you this frustration and effort by reporting an error when it detects this code during compilation. Detecting type safety violations at compile time is the benefit of using generics. Generic Types A generic type is a class or interface that introduces a family of parameterized types by declaring a formal type parameter list (a comma-separated list of type parameter names between angle brackets). This syntax is expressed as follows: class identifier {} interface identifier {} For example, List is a generic type, where List is an interface and type parameter E identifies the list’s element type. Similarly, Map is a generic type, where Map is an interface and type parameters K and V identify the map’s key and value types. NOTE: When declaring a generic type, it is conventional to specify single uppercase letters as type parameter names. Furthermore, these names should be meaningful. For example, E indicates element, T indicates type, K indicates key, and V indicates value. If possible, you should avoid choosing a type parameter name that is meaningless where it is used. For example, List means list of elements, but what does List mean? Parameterized types instantiate generic types. Each parameterized type replaces the generic type’s type parameters with type names. For example, List (List of CHAPTER 5: Mastering Advanced Language Features Part 2 203 Employee) and List (List of String) are examples of parameterized types based on List. Similarly, Map is an example of a parameterized type based on Map. The type name that replaces a type parameter is known as an actual type argument. Generics supports five kinds of actual type arguments:  Concrete type: The name of a class or interface is passed to the type parameter. For example, List employees; specifies that the list elements are Employee instances.  Concrete parameterized type: The name of a parameterized type is passed to the type parameter. For example, List> nameLists; specifies that the list elements are lists of strings.  Array type: An array is passed to the type parameter. For example, List countries; specifies that the list elements are arrays of Strings, possibly city names.  Type parameter: A type parameter is passed to the type parameter. For example, given class declaration class X { List queue; }, X’s type parameter E is passed to List’s type parameter E.  Wildcard: The ? is passed to the type parameter. For example, List list; specifies that the list elements are unknown. You will learn about this type parameter later in the chapter, in “The Need for Wildcards” section. A generic type also identifies a raw type, which is a generic type without its type parameters. For example, List’s raw type is List. Raw types are nongeneric and can hold any Object. NOTE: Java allows raw types to be intermixed with generic types to support the vast amount of legacy code that was written prior to the arrival of generics. However, the compiler outputs a warning message whenever it encounters a raw type in source code. Declaring and Using Your Own Generic Types It is not difficult to declare your own generic types. In addition to specifying a formal type parameter list, your generic type specifies its type parameter(s) throughout its implementation. For example, Listing 5–24 declares a Queue generic type. Listing 5–24. Declaring and using a Queue generic type public class Queue { private E[] elements; private int head, tail; @SuppressWarnings("unchecked") public Queue(int size) CHAPTER 5: Mastering Advanced Language Features Part 2 204 { elements = (E[]) new Object[size]; head = 0; tail = 0; } public void insert(E element) { if (isFull()) // insert() should throw an exception when full. I did return; // not implement insert() to do so for brevity. elements[tail] = element; tail = (tail+1)%elements.length; } public E remove() { if (isEmpty()) return null; E element = elements[head]; head = (head+1)%elements.length; return element; } public boolean isEmpty() { return head == tail; } public boolean isFull() { return (tail+1)%elements.length == head; } public static void main(String[] args) { Queue queue = new Queue(5); System.out.println(queue.isEmpty()); queue.insert("A"); queue.insert("B"); queue.insert("C"); queue.insert("D"); queue.insert("E"); System.out.println(queue.isFull()); System.out.println(queue.remove()); queue.insert("F"); while (!queue.isEmpty()) System.out.println(queue.remove()); System.out.println(queue.isEmpty()); System.out.println(queue.isFull()); } } Queue implements a queue, a data structure that stores elements in first-in, first-out order. An element is inserted at the tail and removed at the head. The queue is empty when the head equals the tail, and full when the tail is one less than the head. Notice that Queue’s E type parameter appears throughout the source code. For example, E appears in the elements array declaration to denote the array’s element type. E is also specified as the type of insert()’s parameter and as remove()’s return type. This book was purchased by harshamv@variable3.comCHAPTER 5: Mastering Advanced Language Features Part 2 205 E also appears in elements = (E[]) new Object[size];. (I will explain later why I specified this expression instead of specifying the more compact elements = new E[size]; expression.) The E[] cast results in the compiler warning about this cast being unchecked. The compiler is concerned that downcasting from Object[] to E[] might result in a violation of type safety because any kind of object can be stored in Object[]. The compiler’s concern is not justified in this example. There is no way that a non-E object can appear in the E[] array. Because the warning is meaningless in this context, it is suppressed by prefixing the constructor with @SuppressWarnings("unchecked"). CAUTION: Be careful when suppressing an unchecked warning. You must first prove that a ClassCastException cannot occur, and then you can suppress the warning. When you run this application, it generates the following output: true true A B C D F true false Type Parameter Bounds List’s E type parameter and Map’s K and V type parameters are examples of unbounded type parameters. You can pass any actual type argument to an unbounded type parameter. It is sometimes necessary to restrict the kinds of actual type arguments that can be passed to a type parameter. For example, you might want to declare a class whose instances can only store instances of classes that subclass an abstract Shape class. To restrict actual type arguments, you can specify an upper bound, a type that serves as an upper limit on the types that can be chosen as actual type arguments. The upper bound is specified via reserved word extends followed by a type name. For example, ShapesList identifies Shape as an upper bound. You can specify ShapesList, ShapesList, and even ShapesList, but not ShapesList because String is not a subclass of Shape. You can assign more than one upper bound to a type parameter, where the first bound is a class or interface, and where each additional upper bound is an interface, by using the ampersand character (&) to separate bound names. Consider Listing 5–25. CHAPTER 5: Mastering Advanced Language Features Part 2 206 Listing 5–25. Assigning multiple upper bounds to a type parameter abstract class Shape { } class Circle extends Shape implements Comparable { private double x, y, radius; Circle(double x, double y, double radius) { this.x = x; this.y = y; this.radius = radius; } @Override public int compareTo(Circle circle) { if (radius < circle.radius) return -1; else if (radius > circle.radius) return 1; else return 0; } @Override public String toString() { return "(" + x + ", " + y + ", " + radius + ")"; } } class SortedShapesList> { @SuppressWarnings("unchecked") private S[] shapes = (S[]) new Shape[2]; private int index = 0; void add(S shape) { shapes[index++] = shape; if (index < 2) return; System.out.println("Before sort: " + this); sort(); System.out.println("After sort: " + this); } private void sort() { if (index == 1) return; if (shapes[0].compareTo(shapes[1]) > 0) { S shape = (S) shapes[0]; shapes[0] = shapes[1]; shapes[1] = shape; } } @Override public String toString() CHAPTER 5: Mastering Advanced Language Features Part 2 207 { return shapes[0].toString() + " " + shapes[1].toString(); } } public class SortedShapesListDemo { public static void main(String[] args) { SortedShapesList ssl = new SortedShapesList(); ssl.add(new Circle(100, 200, 300)); ssl.add(new Circle(10, 20, 30)); } } Listing 5–25’s Circle class extends Shape and implements the java.lang.Comparable interface, which is used to specify the natural ordering of Circle objects. The interface’s compareTo() method implements this ordering by returning a value to reflect the order:  A negative value is returned if the current object should precede the object passed to compareTo() in some fashion.  A zero value is returned if the current and argument objects are the same.  A positive value is returned if the current object should succeed the argument object. Circle’s overriding compareTo() method compares two Circle objects based on their radii. This method orders a Circle instance with the smaller radius before a Circle instance with a larger radius. The SortedShapesList class specifies > as its parameter list. The actual type argument passed to the S parameter must subclass Shape, and it must also implement the Comparable interface. Circle satisfies both criteria: it subclasses Shape and implements Comparable. As a result, the compiler does not report an error when it encounters the main() method’s SortedShapesList ssl = new SortedShapesList(); statement. An upper bound offers extra static type checking that guarantees that a parameterized type adheres to its bounds. This assurance means that the upper bound’s methods can be called safely. For example, sort() can call Comparable’s compareTo() method. If you run this application, you will discover the following output, which shows that the two Circle objects are sorted in ascending order of radius: Before sort: (100.0, 200.0, 300.0) (10.0, 20.0, 30.0) After sort: (10.0, 20.0, 30.0) (100.0, 200.0, 300.0) You can also restrict actual type arguments by specifying a lower bound, a type that serves as a lower limit on the types that can be chosen as actual type arguments. The lower bound is specified via the wildcard, reserved word super, and a type name. Because lower bounds are used exclusively with the wildcard type argument, I will have more to say about lower bounds when I discuss the need for wildcards. CHAPTER 5: Mastering Advanced Language Features Part 2 208 Type Parameter Scope A type parameter’s scope (visibility) is its generic type, which includes the formal type parameter list of which the type parameter is a member. For example, the scope of S in SortedShapesList> is all of SortedShapesList and the formal type parameter list. NOTE: A type parameter bound that includes the type parameter is known as a recursive type bound. For example, Comparable in > is a recursive type bound. Recursive type bounds are rare and typically show up in conjunction with the Comparable interface, for specifying a type’s natural ordering. It is possible to mask a type parameter by declaring a same-named type parameter in a nested type’s formal type parameter list. For example, Listing 5–26 masks an enclosing class’s T type parameter. Listing 5–26. Masking a type variable class EnclosingClass { static class EnclosedClass> { } } EnclosingClass’s T type parameter is masked by EnclosedClass’s T type parameter, which specifies an upper bound where only those types that implement the Comparable interface can be passed to EnclosedClass. If masking is undesirable, it is best to choose a different name for the type parameter. For example, you might specify EnclosedClass>. Although U is not as meaningful a name as T, this situation justifies this choice. The Need for Wildcards In Chapter 3, you learned that a subtype is a kind of supertype. For example, Circle is a kind of Shape and String is a kind of Object. This polymorphic behavior also applies to related parameterized types with the same type parameters (List is a kind of Collection, for example). However, this polymorphic behavior does not apply to multiple parameterized types that differ only in regard to one type parameter being a subtype of another type parameter. For example, List is not a kind of List. Listing 5–27 reveals why parameterized types differing only in type parameters are not polymorphic. Listing 5–27. Proving that parameterized types differing only in type parameters are not polymorphic public static void main(String[] args) { List ls = new ArrayList(); List lo = ls; CHAPTER 5: Mastering Advanced Language Features Part 2 209 lo.add(new Employee()); String s = ls.get(0); } If Listing 5–27 compiled, a ClassCastException would be thrown at runtime. After instantiating a List of String and upcasting its reference to a List of Object, main() adds a new Employee object to the List of Object. The Employee object is then returned via get() and the List of String reference variable. The ClassCastException is thrown because of the implicit cast to String—an Employee is not a String. NOTE: Although you cannot upcast List to List, you can upcast List to the raw type List in order to interoperate with legacy code. This example can be generalized into the following rule: for a given subtype x of type y, and given G as a raw type declaration, G is not a subtype of G. Listing 5–28 shows you how easy it is to run afoul of this rule. Listing 5–28. Attempting to output a list of string public static void main(String[] args) { List ls = new ArrayList(); ls.add("first"); ls.add("second"); ls.add("third"); outputList(ls); } static void outputList(List list) { for (int i = 0; i < list.size(); i++) System.out.println(list.get(i)); } Listing 5–28 will not compile because it assumes that List of String is also a List of Object, which you have just learned is not true because it violates type safety. The outputList() method can only output a list of objects, which makes it useless. The wildcard type argument (?) provides a typesafe workaround to this problem because it accepts any actual type argument. Simply change List list to List list and Listing 5–28 will compile. However, you cannot add elements to a List because doing so would violate type safety. For example, Listing 5–29 presents a copyList() method that attempts to copy one List to another List. Listing 5–29. Attempting to copy one List to another List public static void main(String[] args) { List ls1 = new ArrayList(); ls1.add("first"); ls1.add("second"); ls1.add("third"); List ls2 = new ArrayList(); CHAPTER 5: Mastering Advanced Language Features Part 2 210 copyList(ls1, ls2); } static void copyList(List list1, List list2) { for (int i = 0; i < list1.size(); i++) list2.add(list1.get(i)); } Listing 5–29 will not compile. Instead, the compiler reports the following error message when it encounters list2.add(list1.get(i));: x.java:13: cannot find symbol symbol : method add(java.lang.Object) location: interface java.util.List list2.add(list1.get(i)); ^ 1 error The error message reflects that although list1’s elements (which can be of any type) can be assigned to Object, list2’s element type is unknown. If this type is anything other than Object (such as String), type safety is violated. One solution to this problem is to specify static void copyList(List list1, List list2). A second solution to this problem requires that copyList()’s first type parameter be specified as ? extends String and its second parameter be specified as ? super String. Listing 5–30 reveals the same copyList() method with these upper and lower bounds. Listing 5–30. Using bounded wildcards to successfully copy one list to another public static void main(String[] args) { List ls1 = new ArrayList(); ls1.add("first"); ls1.add("second"); ls1.add("third"); List ls2 = new ArrayList(); copyList(ls1, ls2); } static void copyList(List list1, List list2) { for (int i = 0; i < list1.size(); i++) list2.add(list1.get(i)); } Listing 5–30’s copyList() method reveals that the list1 parameter accepts String or any subclass. Because String is declared final, no subclasses can be passed as actual type arguments. In contrast, the list2 parameter accepts String and its Object superclass—a subclass is a kind of superclass. Although Listing 5–30 compiles and runs, its version of the copyList() header is little better than specifying static void copyList(List list1, List list2). You can only copy a List of String to a List of String or a List of Object. You will shortly discover that generic methods offer a much better solution. CHAPTER 5: Mastering Advanced Language Features Part 2 211 Reification and Erasure Reification is the process or result of treating the abstract as if it was concrete. For example, 0xa000000 is an abstract hexadecimal integer literal that is treated as if it was the concrete 32-bit memory address that it represents. Java arrays are reified. Because they are aware of their element types, they can enforce these types at runtime. Attempting to store an invalid element in an array (see Listing 5– 31) results in a java.lang.ArrayStoreException. Listing 5–31. How an ArrayStoreException occurs class Point { int x, y; } class ColoredPoint extends Point { int color; } public class ReificationDemo { public static void main(String[] args) { ColoredPoint[] cptArray = new ColoredPoint[1]; Point[] ptArray = cptArray; ptArray[0] = new Point(); } } Listing 5–31 demonstrates that arrays are covariant in that an array of supertype references is a supertype of an array of subtype references. A subtype array can be assigned to a supertype variable; for example, Point[] ptArray = cptArray;. Covariance can be dangerous. For example, an ArrayStoreException is thrown when ptArray[0] = new Point(); tries to store a Point object in the ColoredPoint array via the ptArray intermediary. The exception occurs because a Point is not a ColoredPoint. In contrast to arrays, a generic type’s type parameters are not reified. In other words, they are not available at runtime. Instead, these type parameters are discarded following compilation. This throwing away of type parameters is known as erasure. Erasure also involves replacing uses of other type variables by the upper bound of the type variable (such as Object) and inserting casts to the appropriate type when the resulting code is not type correct. Although erasure makes it possible for generic types to interoperate with raw types, it does have consequences. For example, you cannot create an array of a generic type: new List[10], new E[size], and new Map[100] are illegal. The compiler reports a “generic array creation” error when it encounters an attempt to create an array of a generic type. It does so because the attempt is not typesafe. If allowed, compiler-inserted casts could trigger ClassCastExceptions—see Listing 5–32. CHAPTER 5: Mastering Advanced Language Features Part 2 212 Listing 5–32. Why creating an array of a generic type is not a good idea List[] empListArray = new List[1]; List strList = new ArrayList(); strList.add("string"); Object[] objArray = empListArray; objArray[0] = strList; Employee e = empListArray[0].get(0); Let us assume that Listing 5–32 is legal. The first line creates a one-element array where this element stores a List of Employee. The second line creates a List of String and stores a single String in this list. The third line assigns empListArray to objArray. This assignment is legal because arrays are covariant. The fourth line stores the List of String into objArray[0], which works because of erasure. An ArrayStoreException does not occur because List’s runtime type is List and List[]’s runtime type is List[]. However, there is a problem. A List instance has been stored in an array that can only hold List instances. When the compiler-inserted cast operator attempts to cast empListArray[0].get(0)’s return value ("string") to Employee, the cast operator throws a ClassCastException object. Another consequence of erasure is that the instanceof operator cannot be used with parameterized types apart from unbounded wildcard types. For example, List ls = null; if (ls instanceof LinkedList) {} is illegal. Instead you must change the instanceof expression to either of the following expressions: ls instanceof LinkedList // legal to use with unbounded wildcard type ls instanceof LinkedList // legal to use with raw type (the preferred use) Generic Methods A generic method is a static or non-static method with a type-generalized implementation. A formal type parameter list precedes the method’s return type, uses the same syntax, and has the same meaning as the generic type’s formal type parameter list. This syntax is expressed as follows: return_type identifier(parameter_list) { } The collections framework provides many examples of generic methods. For example, the Collections class provides a public static > T min(Collection coll) method for returning the minimum element in the given collection according to the natural ordering of its elements. Listing 5–33 converts the aforementioned copyList() method into a generic method so that it can copy a list of an arbitrary type to another list of the same type. Listing 5–33. Declaring and using a copyList() generic method class Circle { private double x, y, radius; CHAPTER 5: Mastering Advanced Language Features Part 2 213 Circle(double x, double y, double radius) { this.x = x; this.y = y; this.radius = radius; } @Override public String toString() { return "(" + x + ", " + y + ", " + radius + ")"; } } public class CopyList { public static void main(String[] args) { List ls = new ArrayList(); ls.add("A"); ls.add("B"); ls.add("C"); outputList(ls); List lsCopy = new ArrayList(); copyList(ls, lsCopy); outputList(lsCopy); List lc = new ArrayList(); lc.add(new Circle(10.0, 20.0, 30.0)); lc.add(new Circle (5.0, 4.0, 16.0)); outputList(lc); List lcCopy = new ArrayList(); copyList(lc, lcCopy); outputList(lcCopy); } static void copyList(List c1, List c2) { for (int i = 0; i < c1.size(); i++) c2.add(c1.get(i)); } static void outputList(List l) { for (int i = 0; i < l.size(); i++) System.out.println (l.get(i)); System.out.println(); } } The generic method’s type parameters are inferred from the context in which the method was invoked. For example, the compiler determines that copyList(ls, lsCopy); copies a List of String to another List of String. Similarly, it determines that copyList(lc, lcCopy); copies a List of Circle to another List of Circle. CHAPTER 5: Mastering Advanced Language Features Part 2 214 When you run this application, it generates the following output: A B C A B C (10.0, 20.0, 30.0) (5.0, 4.0, 16.0) (10.0, 20.0, 30.0) (5.0, 4.0, 16.0) Enums An enumerated type is a type that specifies a named sequence of related constants as its legal values. The months in a calendar, the coins in a currency, and the days of the week are examples of enumerated types. Java developers have traditionally used sets of named integer constants to represent enumerated types. Because this form of representation has proven to be problematic, Java version 5 introduced the enum alternative. This section introduces you to enums. After discussing the problems with traditional enumerated types, the section presents the enum alternative. It then introduces you to the Enum class, from which enums originate. The Trouble with Traditional Enumerated Types Listing 5–34 declares a Coin enumerated type whose set of constants identifies different kinds of coins in a currency. Listing 5–34. An enumerated type identifying coins public class Coin { public final static int PENNY = 0; public final static int NICKEL = 1; public final static int DIME = 2; public final static int QUARTER = 3; } Listing 5–35 declares a Weekday enumerated type whose set of constants identifies the days of the week. Listing 5–35. An enumerated type identifying weekdays public class Weekday { public final static int SUNDAY = 0; public final static int MONDAY = 1; This book was purchased by harshamv@variable3.comCHAPTER 5: Mastering Advanced Language Features Part 2 215 public final static int TUESDAY = 2; public final static int WEDNESDAY = 3; public final static int THURSDAY = 4; public final static int FRIDAY = 5; public final static int SATURDAY = 6; } Listing 5–34’s and 5–35’s approach to representing an enumerated type is problematic, where the biggest problem is the lack of compile-time type safety. For example, you can pass a coin to a method that requires a weekday and the compiler will not complain. You can also compare coins to weekdays, as in Coin.NICKEL == Weekday.MONDAY, and specify even more meaningless expressions, such as Coin.DIME+Weekday.FRIDAY- 1/Coin.QUARTER. The compiler does not complain because it only sees ints. Applications that depend upon enumerated types are brittle. Because the type’s constants are compiled into an application’s classfiles, changing a constant’s int value requires you to recompile dependent applications or risk them behaving erratically. Another problem with enumerated types is that int constants cannot be translated into meaningful string descriptions. For example, what does 4 mean when debugging a faulty application? Being able to see THURSDAY instead of 4 would be more helpful. NOTE: You could circumvent the previous problem by using String constants. For example, you might specify public final static String THURSDAY = "THURSDAY";. Although the constant value is more meaningful, String-based constants can impact performance because you cannot use == to efficiently compare just any old strings (as you will discover in Chapter 7). Other problems related to String-based constants include hard-coding the constant’s value ("THURSDAY") instead of the constant’s name (THURSDAY) into source code, which makes it very difficult to change the constant’s value at a later time; and misspelling a hard-coded constant ("THURZDAY"), which compiles correctly but is problematic at runtime. The Enum Alternative Java version 5 introduced enums as a better alternative to traditional enumerated types. An enum is an enumerated type that is expressed via reserved word enum. Listing 5–36 uses enum to declare Listing 5–34’s and 5–35’s enumerated types. Listing 5–36. Improved enumerated types for coins and weekdays public enum Coin { PENNY, NICKEL, DIME, QUARTER } public enum Weekday { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY } Despite their similarity to the int-based enumerated types found in C++ and other languages, Listing 5–36’s enums are classes. Each constant is a public static final field that represents an instance of its enum class. CHAPTER 5: Mastering Advanced Language Features Part 2 216 Because constants are final, and because you cannot call an enum’s constructors to create more constants, you can use == to compare constants efficiently and (unlike string constant comparisons) safely. For example, you can specify c == Coin.NICKEL. Enums promote compile-time type safety by preventing you from comparing constants in different enums. For example, the compiler will report an error when it encounters Coin.PENNY == Weekday.SUNDAY. The compiler also frowns upon passing a constant of the wrong enum kind to a method. For example, you cannot pass Weekday.FRIDAY to a method whose parameter type is Coin. Applications depending upon enums are not brittle because the enum’s constants are not compiled into an application’s classfiles. Also, the enum provides a toString() method for returning a more useful description of a constant’s value. Because enums are so useful, Java version 5 enhanced the switch statement to support them. Listing 5–37 demonstrates this statement switching on one of the constants in Listing 5–36’s Coin enum. Listing 5–37. Using the switch statement with an enum public class EnhancedSwitch { private enum Coin { PENNY, NICKEL, DIME, QUARTER } public static void main(String[] args) { Coin coin = Coin.NICKEL; switch (coin) { case PENNY : System.out.println("1 cent"); break; case NICKEL : System.out.println("5 cents"); break; case DIME : System.out.println("10 cents"); break; case QUARTER: System.out.println("25 cents"); default : assert false; } } } Listing 5–37 demonstrates switching on an enum’s constants. This enhanced statement only allows you to specify the name of a constant as a case label. If you prefix the name with the enum, as in case Coin.DIME, the compiler reports an error. Enhancing an Enum You can add fields, constructors, and methods to an enum—you can even have the enum implement interfaces. For example, Listing 5–38 adds a field, a constructor, and two methods to Coin to associate a denomination value with a Coin constant (such as 1 for penny and 5 for nickel) and convert pennies to the denomination. CHAPTER 5: Mastering Advanced Language Features Part 2 217 Listing 5–38. Enhancing the Coin enum public enum Coin { PENNY(1), NICKEL(5), DIME(10), QUARTER(25); private final int denomValue; Coin(int denomValue) { this.denomValue = denomValue; } public int denomValue() { return denomValue; } public int toDenomination(int numPennies) { return numPennies/denomValue; } } Listing 5–38’s constructor accepts a denomination value, which it assigns to a private blank final field named denomValue—all fields should be declared final because constants are immutable. Notice that this value is passed to each constant during its creation (PENNY(1), for example). CAUTION: When the comma-separated list of constants is followed by anything other than an enum’s closing brace, you must terminate the list with a semicolon or the compiler will report an error. Furthermore, this listing’s denomValue() method returns denomValue, and its toDenomination() method returns the number of coins of that denomination that are contained within the number of pennies passed to this method as its argument. For example, 3 nickels are contained in 16 pennies. Listing 5–39 shows you how to use the enhanced Coin enum. Listing 5–39. Exercising the enhanced Coin enum public static void main(String[] args) { if (args.length == 1) { int numPennies = Integer.parseInt(args[0]); System.out.println(numPennies + " pennies is equivalent to:"); int numQuarters = Coin.QUARTER.toDenomination(numPennies); System.out.println(numQuarters + " " + Coin.QUARTER.toString() + (numQuarters != 1 ? "s," : ",")); numPennies -= numQuarters*Coin.QUARTER.denomValue(); int numDimes = Coin.DIME.toDenomination(numPennies); System.out.println(numDimes + " " + Coin.DIME.toString() + CHAPTER 5: Mastering Advanced Language Features Part 2 218 (numDimes != 1 ? "s, " : ",")); numPennies -= numDimes*Coin.DIME.denomValue(); int numNickels = Coin.NICKEL.toDenomination(numPennies); System.out.println(numNickels + " " + Coin.NICKEL.toString() + (numNickels != 1 ? "s, " : ", and")); numPennies -= numNickels*Coin.NICKEL.denomValue(); System.out.println(numPennies + " " + Coin.PENNY.toString() + (numPennies != 1 ? "s" : "")); } System.out.println(); System.out.println("Denomination values:"); for (int i = 0; i < Coin.values().length; i++) System.out.println(Coin.values()[i].denomValue()); } Listing 5–39 describes an application that converts its solitary “pennies” command-line argument to an equivalent amount expressed in quarters, dimes, nickels, and pennies. In addition to calling a Coin constant’s denomValue() and toDenomValue() methods, the application calls toString() to output a string representation of the coin. Another called enum method is values(). This method returns an array of all Coin constants that are declared in the Coin enum (value()’s return type, in this example, is Coin[]). This array is useful when you need to iterate over these constants. For example, Listing 5–39 calls this method to output each coin’s denomination. When you run this application with 119 as its command-line argument, it generates the following output: 119 pennies is equivalent to: 4 QUARTERs, 1 DIME, 1 NICKEL, and 4 PENNYs Denomination values: 1 5 10 25 The output shows that toString() returns a constant’s name. It is sometimes useful to override this method to return a more meaningful value. For example, a method that extracts tokens (named character sequences) from a string might use a Token enum to list token names and, via an overriding toString() method, values—see Listing 5–40. Listing 5–40. Overriding toString() to return a Token constant’s value public enum Token { IDENTIFIER("ID"), INTEGER("INT"), LPAREN("("), RPAREN(")"), COMMA(","); private final String tokValue; Token(String tokValue) CHAPTER 5: Mastering Advanced Language Features Part 2 219 { this.tokValue = tokValue; } @Override public String toString() { return tokValue; } public static void main(String[] args) { System.out.println("Token values:"); for (int i = 0; i < Token.values().length; i++) System.out.println(Token.values()[i].name() + " = " + Token.values()[i]); } } This application calls values() to return the array of Token constants. For each constant, it calls the constant’s name() method to return the constant’s name, and implicitly calls toString() to return the constant’s value. If you were to run this application, you would observe the following output: Token values: IDENTIFIER = ID INTEGER = INT LPAREN = ( RPAREN = ) COMMA = , Another way to enhance an enum is to assign a different behavior to each constant. You can accomplish this task by introducing an abstract method into the enum and overriding this method in an anonymous subclass of the constant. Listing 5–41’s TempConversion enum demonstrates this technique. Listing 5–41. Using anonymous subclasses to vary the behaviors of enum constants public enum TempConversion { C2F("Celsius to Fahrenheit") { @Override public double convert(double value) { return value*9.0/5.0+32.0; } }, F2C("Fahrenheit to Celsius") { @Override public double convert(double value) { return (value-32.0)*5.0/9.0; } }; TempConversion(String desc) { CHAPTER 5: Mastering Advanced Language Features Part 2 220 this.desc = desc; } private String desc; @Override public String toString() { return desc; } public abstract double convert(double value); public static void main(String[] args) { System.out.println(C2F + " for 100.0 degrees = " + C2F.convert(100.0)); System.out.println(F2C + " for 98.6 degrees = " + F2C.convert(98.6)); } } When you run this application, it generates the following output: Celsius to Fahrenheit for 100.0 degrees = 212.0 Fahrenheit to Celsius for 98.6 degrees = 37.0 The Enum Class The compiler regards enum as syntactic sugar. When it encounters an enum type declaration (enum Coin {}), it generates a class whose name (Coin) is specified by the declaration, and which also subclasses the abstract Enum class (in the java.lang package), the common base class of all Java language–based enumeration types. If you examine Enum’s Java documentation, you will discover that it overrides Object’s clone(), equals(), finalize(), hashCode(), and toString() methods:  clone() is overridden to prevent constants from being cloned so that there is never more than one copy of a constant; otherwise, constants could not be compared via ==.  equals() is overridden to compare constants via their references— constants with the same identities (==) must have the same contents (equals()), and different identities imply different contents.  finalize() is overridden to ensure that constants cannot be finalized.  hashCode() is overridden because equals() is overridden.  toString() is overridden to return the constant’s name. Except for toString(), all of the overridden methods are declared final so that they cannot be overridden in a subclass. Enum also provides its own methods. These methods include the final compareTo(), (Enum implements Comparable), getDeclaringClass(), name(), and ordinal() methods: CHAPTER 5: Mastering Advanced Language Features Part 2 221  compareTo() compares the current constant with the constant passed as an argument to see which constant precedes the other constant in the enum, and returns a value indicating their order. This method makes it possible to sort an array of unsorted constants.  getDeclaringClass() returns the Class object corresponding to the current constant’s enum. For example, the Class object for Coin is returned when calling Coin.PENNY.getDeclaringClass() for Listing 5– 36’s Coin enum. Also, TempConversion is returned when calling TempConversion.C2F.getDeclaringClass() for Listing 5–41’s TempConversion enum. The compareTo() method uses Class’s getClass() method and Enum’s getDeclaringClass() method to ensure that only constants belonging to the same enum are compared. Otherwise, a ClassCastException is thrown. (I will discuss Class in Chapter 7.)  name() returns the constant’s name. Unless overridden to return something more descriptive, toString() also returns the constant’s name.  ordinal() returns a zero-based ordinal, an integer that identifies the position of the constant within the enum type. compareTo() compares ordinals. Enum also provides the static valueOf(ClassenumType, String name) method for returning the enum constant from the specified enum with the specified name:  enumType identifies the Class object of the enum from which to return a constant.  name identifies the name of the constant to return. For example, Coin penny = Enum.valueOf(Coin.class, "PENNY"); assigns the Coin constant whose name is PENNY to penny. You will not discover a values() method in Enum’s Java documentation because the compiler synthesizes (manufactures) this method while generating the class. Extending the Enum Class Enum’s generic type is Enum>. Although the formal type parameter list looks ghastly, it is not that hard to understand. But first, take a look at Listing 5–42. Listing 5–42. The Coin class as it appears from the perspective of its classfile public final class Coin extends Enum { public static final Coin PENNY = new Coin("PENNY", 0); public static final Coin NICKEL = new Coin("NICKEL", 1); public static final Coin DIME = new Coin("DIME", 2); public static final Coin QUARTER = new Coin("QUARTER", 3); private static final Coin[] $VALUES = { PENNY, NICKEL, DIME, QUARTER }; public static Coin[] values() CHAPTER 5: Mastering Advanced Language Features Part 2 222 { return Coin.$VALUES.clone(); } public static Coin valueOf(String name) { return Enum.valueOf(Coin.class, "Coin"); } private Coin(String name, int ordinal) { super(name, ordinal); } } Behind the scenes, the compiler converts Listing 5–36’s Coin enum declaration into a class declaration that is similar to Listing 5–42. The following rules show you how to interpret Enum> in the context of Coin extends Enum:  Any subclass of Enum must supply an actual type argument to Enum. For example, Coin’s header specifies Enum.  The actual type argument must be a subclass of Enum. For example, Coin is a subclass of Enum.  A subclass of Enum (such as Coin) must follow the idiom that it supplies its own name (Coin) as an actual type argument. The third rule allows Enum to declare methods—compareTo(), getDeclaringClass(), and valueOf()—whose parameter and/or return types are specified in terms of the subclass (Coin), and not in terms of Enum. The rationale for doing this is to avoid having to specify casts. For example, you do not need to cast valueOf()’s return value to Coin in Coin penny = Enum.valueOf(Coin.class, "PENNY");. NOTE: You cannot compile Listing 5–42 because the compiler will not compile any class that extends Enum. It will also complain about super(name, ordinal);. EXERCISES The following exercises are designed to test your understanding of assertions, annotations, generics, and enums: 1. What is an assertion? 2. When would you use assertions? 3. True or false: Specifying the -ea command-line option with no argument enables all assertions, including system assertions. 4. What is an annotation? CHAPTER 5: Mastering Advanced Language Features Part 2 223 5. What kinds of application elements can be annotated? 6. Identify the three compiler-supported annotation types. 7. How do you declare an annotation type? 8. What is a marker annotation? 9. What is an element? 10. How do you assign a default value to an element? 11. What is a meta-annotation? 12. Identify Java’s four meta-annotation types. 13. Define generics. 14. Why would you use generics? 15. What is the difference between a generic type and a parameterized type? 16. Which one of the nonstatic member class, local class, and anonymous class inner class categories cannot be generic? 17. Identify the five kinds of actual type arguments. 18. True or false: You cannot specify a primitive type name (such as double or int) as an actual type argument. 19. What is a raw type? 20. When does the compiler report an unchecked warning message and why? 21. How do you suppress an unchecked warning message? 22. True or false: List’s E type parameter is unbounded. 23. How do you specify a single upper bound? 24. True or false: MyList specifies that the E type parameter has a lower bound of Circle. 25. What is a recursive type bound? 26. Why are wildcard type arguments necessary? 27. What is reification? 28. True or false: Type parameters are reified. 29. What is erasure? 30. What is a generic method? 31. In Listing 5–43, which overloaded method does the methodCaller() generic method call? Listing 5–43. Which someOverloadedMethod() is called? import java.util.Date; public class CallOverloadedNGMethodFromGMethod { xCHAPTER 5: Mastering Advanced Language Features Part 2 224 public static void someOverloadedMethod(Object o) { System.out.println("call to someOverloadedMethod(Object o)"); } public static void someOverloadedMethod(Date d) { System.out.println("call to someOverloadedMethod(Date d)"); } public static void methodCaller(T t) { someOverloadedMethod(t); } public static void main(String[] args) { methodCaller(new Date()); } } 32. What is an enumerated type? 33. Identify three problems that can arise when you use enumerated types whose constants are int-based. 34. What is an enum? 35. How do you use the switch statement with an enum? 36. In what ways can you enhance an enum? 37. What is the purpose of the abstract Enum class? 38. What is the difference between Enum’s name() and toString() methods? 39. True or false: Enum’s generic type is Enum>. 40. Declare a ToDo marker annotation type that annotates only type elements, and that also uses the default retention policy. 41. Rewrite the StubFinder application to work with Listing 5–15’s Stub annotation type (with appropriate @Target and @Retention annotations) and Listing 5–16’s Deck class. 42. Implement a stack in a manner that is similar to Listing 5–24’s Queue class. Stack must be generic, it must declare push(), pop(), and isEmpty() methods (it could also declare an isFull() method but that method is not necessary in this exercise), push() must throw a StackFullException instance when the stack is full, and pop() must throw a StackEmptyException instance when the stack is empty. (You must create your own package-private StackFullException and StackEmptyException helper classes because they are not provided for you in Java’s class library.) Declare a similar main() method, and insert two assertions into this method that validate your assumptions about the stack being empty immediately after being created and immediately after popping the last element. This book was purchased by harshamv@variable3.comCHAPTER 5: Mastering Advanced Language Features Part 2 225 NOTE: A stack is a data structure that stores elements in a last-in, first-out order. Elements are added to the stack via an operation known as push. They are removed from the stack via an operation known as pop. The last element pushed onto the stack is the first element popped off of the stack. 43. Declare a Compass enum with NORTH, SOUTH, EAST, and WEST members. Declare a UseCompass class whose main() method randomly selects one of these constants and then switches on that constant. Each of the switch statement’s cases should output a message such as heading north. Summary An assertion is a statement that lets you express an assumption of application correctness via a Boolean expression. If this expression evaluates to true, execution continues with the next statement. Otherwise, an error that identifies the cause of failure is thrown. There are many situations where assertions should be used. These situations organize into internal invariant, control-flow invariant, and design-by-contract categories. An invariant is something that does not change. Although there are many situations where assertions should be used, there also are situations where they should be avoided. For example, you should not use assertions to check the arguments that are passed to public methods. The compiler records assertions in the classfile. However, assertions are disabled at runtime because they can affect performance. You must enable the classfile’s assertions before you can test assumptions about the behaviors of your classes. Annotations are instances of annotation types and associate metadata with application elements. They are expressed in source code by prefixing their type names with @ symbols. For example, @Readonly is an annotation and Readonly is its type. Java supplies a wide variety of annotation types, including the compiler-oriented Override, Deprecated, and SuppressWarnings types. However, you can also declare your own annotation types by using the @interface syntax. Annotation types can be annotated with meta-annotations that identify the application elements they can target (such as constructors, methods, or fields), their retention policies, and other characteristics. Annotations whose types are assigned a runtime retention policy via @Retention annotations can be processed at runtime using custom applications or Java’s apt tool, whose functionality has been integrated into the compiler starting with Java version 6. CHAPTER 5: Mastering Advanced Language Features Part 2 226 Java version 5 introduced generics, language features for declaring and using type- agnostic classes and interfaces. When working with Java’s collections framework, these features help you avoid ClassCastExceptions. A generic type is a class or interface that introduces a family of parameterized types by declaring a formal type parameter list. A generic method is a static or non-static method with a type-generalized implementation. An enumerated type is a type that specifies a named sequence of related constants as its legal values. Java developers have traditionally used sets of named integer constants to represent enumerated types. Because sets of named integer constants have proven to be problematic, Java version 5 introduced the enum alternative. An enum is an enumerated type that is expressed via reserved word enum. You can add fields, constructors, and methods to an enum—you can even have the enum implement interfaces. Also, you can override toString() to provide a more useful description of a constant’s value, and subclass constants to assign different behaviors. The compiler regards enum as syntactic sugar for a class that subclasses Enum. This abstract class overrides various Object methods to provide default behaviors (usually for safety reasons), and provides additional methods for various purposes. This chapter largely completes our tour of the Java language. However, there are a few more advanced language features to explore. You will encounter one of these minor features in Chapter 6, which begins a multichapter exploration of various types that are located in Java SE’s/Android’s standard class library. 227 227 Chapter Exploring the Basic APIs Part 1 Aspiring Android developers need to acquire a solid understanding of foundational Java APIs. You have already encountered a few of these APIs, such as the Object and String classes and the Throwable class hierarchy. This chapter introduces you to additional language-oriented (basic) APIs pertaining to math, packages, primitive types, and the garbage collector. NOTE: Chapter 6 explores basic API classes and interfaces that are located in the java.lang, java.lang.ref, and java.math packages. Math APIs Chapter 2 presented Java’s +, -, *, /, and % operators for performing basic arithmetic on primitive type values. Java also provides classes for performing trigonometry and other advanced math operations, representing monetary values accurately, and supporting extremely long integers for use in RSA encryption (http://en.wikipedia.org/wiki/RSA) and other contexts. Math and StrictMath The java.lang.Math class declares double constants E and PI that represent the natural logarithm base value (2.71828...) and the ratio of a circle’s circumference to its diameter (3.14159...). E is initialized to 2.718281828459045 and PI is initialized to 3.141592653589793. Math also declares assorted class methods to perform various math operations. Table 6–1 describes many of these methods. 6 CHAPTER 6: Exploring the Basic APIs Part 1 228 Table 6–1. Math Methods Method Description double abs(double d) Return the absolute value of d. There are four special cases: abs(-0.0) = +0.0, abs(+infinity) = +infinity, abs(- infinity) = +infinity, and abs(NaN) = NaN. float abs(float f) Return the absolute value of f. There are four special cases: abs(-0.0) = +0.0, abs(+infinity) = +infinity, abs(- infinity) = +infinity, and abs(NaN) = NaN. int abs(int i) Return the absolute value of i. There is one special case: the absolute value of Integer.MIN_VALUE is Integer.MIN_VALUE. long abs(long l) Return the absolute value of l. There is one special case: the absolute value of Long.MIN_VALUE is Long.MIN_VALUE. double acos(double d) Return angle d’s arc cosine within the range 0 through PI. There are three special cases: acos(anything > 1) = NaN, acos(anything < -1) = NaN, and acos(NaN) = NaN. double asin(double d) Return angle d’s arc sine within the range -PI/2 through PI/2. There are three special cases: asin(anything > 1) = NaN, asin(anything < -1) = NaN, and asin(NaN) = NaN. double atan(double d) Return angle d’s arc tangent within the range -PI/2 through PI/2. There are five special cases: atan(+0.0) = +0.0, atan(-0.0) = -0.0, atan(+infinity) = +PI/2, atan(-infinity) = -PI/2, and atan(NaN) = NaN. double ceil(double d) Return the smallest value (closest to negative infinity) that is not less than d and is equal to an integer. There are six special cases: ceil(+0.0) = +0.0, ceil(-0.0) = -0.0, ceil(anything > -1.0 and < 0.0) = -0.0, ceil(+infinity) = +infinity, ceil(- infinity) = -infinity, and ceil(NaN) = NaN. double cos(double d) Return the cosine of angle d (expressed in radians). There are three special cases: cos(+infinity) = NaN, cos(-infinity) = NaN, and cos(NaN) = NaN. double exp(double d) Return Euler’s number e raised to the power d. There are three special cases: exp(+infinity) = +infinity, exp(-infinity) = +0.0, and exp(NaN) = NaN. double floor(double d) Return the largest value (closest to positive infinity) that is not greater than d and is equal to an integer. There are five special cases: floor(+0.0) = +0.0, floor(-0.0) = -0.0, floor(+infinity) = +infinity, floor(-infinity) = -infinity, and floor(NaN) = NaN. CHAPTER 6: Exploring the Basic APIs Part 1 229 Method Description double log(double d) Return the natural logarithm (base e) of d. There are six special cases: log(+0.0) = -infinity, log(-0.0) = -infinity, log(anything < 0) = NaN, log(+infinity) = +infinity, log(- infinity) = NaN, and log(NaN) = NaN. double log10(double d) Return the base 10 logarithm of d. There are six special cases: log10(+0.0) = -infinity, log10(-0.0) = -infinity, log10(anything < 0) = NaN, log10(+infinity) = +infinity, log10(-infinity) = NaN, and log10(NaN) = NaN. double max(double d1, double d2) Return the most positive (closest to positive infinity) of d1 and d2. There are four special cases: max(NaN, anything) = NaN, max(anything, NaN) = NaN, max(+0.0, -0.0) = +0.0, and max(- 0.0, +0.0) = +0.0. float max(double f1, double f2) Return the most positive (closest to positive infinity) of f1 and f2. There are four special cases: max(NaN, anything) = NaN, max(anything, NaN) = NaN, max(+0.0, -0.0) = +0.0, and max(- 0.0, +0.0) = +0.0. int max(int i1, int i2) Return the most positive (closest to positive infinity) of i1 and i2. long max(long l1, long l2) Return the most positive (closest to positive infinity) of l1 and l2. double min(double d1, double d2) Return the most negative (closest to negative infinity) of d1 and d2. There are four special cases: min(NaN, anything) = NaN, min(anything, NaN) = NaN, min(+0.0, -0.0) = -0.0, and min(- 0.0, +0.0) = -0.0. float min(float f1, float f2) Return the most negative (closest to negative infinity) of f1 and f2. There are four special cases: min(NaN, anything) = NaN, min(anything, NaN) = NaN, min(+0.0, -0.0) = -0.0, and min(- 0.0, +0.0) = -0.0. int min(int i1, int i2) Return the most negative (closest to negative infinity) of i1 and i2. long min(long l1, long l2) Return the most negative (closest to negative infinity) of l1 and l2. double random() Return a pseudorandom number between 0.0 (inclusive) and 1.0 (exclusive). long round(double d) Return the result of rounding d to a long integer. The result is equivalent to (long) Math.floor(d+0.5). There are seven special cases: round(+0.0) = +0.0, round(-0.0) = +0.0, round(anything > Long.MAX_VALUE) = Long.MAX_VALUE, round(anything < Long.MIN_VALUE) = Long.MIN_VALUE, CHAPTER 6: Exploring the Basic APIs Part 1 230 Method Description round(+infinity) = Long.MAX_VALUE, round(-infinity) = Long.MIN_VALUE, and round(NaN) = +0.0. int round(float f) Return the result of rounding f to an integer. The result is equivalent to (int) Math.floor(f+0.5). There are seven special cases: round(+0.0) = +0.0, round(-0.0) = +0.0, round(anything > Integer.MAX_VALUE) = Integer.MAX_VALUE, round(anything < Integer.MIN_VALUE) = Integer.MIN_VALUE, round(+infinity) = Integer.MAX_VALUE, round(-infinity) = Integer.MIN_VALUE, and round(NaN) = +0.0. double signum(double d) Return the sign of d as -1.0 (d less than 0.0), 0.0 (d equals 0.0), and 1.0 (d greater than 0.0). There are five special cases: signum(+0.0) = +0.0, signum(-0.0) = -0.0, signum(+infinity) = +1.0, signum(-infinity) = -1.0, and signum(NaN) = NaN. float signum(float f) Return the sign of f as -1.0 (f less than 0.0), 0.0 (f equals 0.0), and 1.0 (f greater than 0.0). There are five special cases: signum(+0.0) = +0.0, signum(-0.0) = -0.0, signum(+infinity) = +1.0, signum(-infinity) = -1.0, and signum(NaN) = NaN. double sin(double d) Return the sine of angle d (expressed in radians). There are five special cases: sin(+0.0) = +0.0, sin(-0.0) = -0.0, sin(+infinity) = NaN, sin(-infinity) = NaN, and sin(NaN) = NaN. double sqrt(double d) Return the square root of d. There are five special cases: sqrt(+0.0) = +0.0, sqrt(-0.0) = -0.0, sqrt(anything < 0) = NaN, sqrt(+infinity) = +infinity, and sqrt(NaN) = NaN. double tan(double d) Return the tangent of angle d (expressed in radians). There are five special cases: tan(+0.0) = +0.0, tan(-0.0) = -0.0, tan(+infinity) = NaN, tan(-infinity) = NaN, and tan(NaN) = NaN. double toDegrees (double angrad) Convert angle angrad from radians to degrees via expression angrad*180/PI. There are five special cases: toDegrees(+0.0) = +0.0, toDegrees(-0.0) = -0.0, toDegrees(+infinity) = +infinity, toDegrees(-infinity) = -infinity, and toDegrees(NaN) = NaN. double toRadians (angdeg) Convert angle angdeg from degrees to radians via expression angdeg/180*PI. There are five special cases: toRadians(+0.0) = +0.0, toRadians(-0.0) = -0.0, toRadians(+infinity) = +infinity, toRadians(-infinity) = -infinity, and toRadians(NaN) = NaN. CHAPTER 6: Exploring the Basic APIs Part 1 231 Table 6–1 reveals a wide variety of useful math-oriented methods. For example, each abs() method returns its argument’s absolute value (number without regard for sign). abs(double) and abs(float) are useful for comparing double precision floating-point and floating-point values safely. For example, 0.3 == 0.1+0.1+0.1 evaluates to false because 0.1 has no exact representation. However, you can compare these expressions with abs() and a tolerance value, which indicates an acceptable range of error. For example, Math.abs(0.3-(0.1+0.1+0.1)) < 0.1 returns true because the absolute difference between 0.3 and 0.1+0.1+0.1 is less than a 0.1 tolerance value. Previous chapters demonstrated other Math methods. For example, Chapter 2 demonstrated Math’s sin(), toRadians(), cos(), round(double), and random() methods. As Chapter 5’s Lotto649 application revealed, random() (which returns a number that appears to be randomly chosen but is actually chosen by a predictable math calculation, and hence is pseudorandom) is useful in simulations, games, and wherever an element of chance is needed, but first its return value (0.0 to almost 1.0) must somehow be transformed into a more useful range, perhaps 0 through 49, or maybe -100 through 100. You will find Listing 6–1’s rnd() method useful for making these transformations. Listing 6–1. Converting random()’s return value into something more useful public static int rnd(int limit) { return (int) (Math.random()*limit); } rnd() transforms random()’s 0.0 to almost 1.0 double precision floating-point range to a 0 through limit - 1 integer range. For example, rnd(50) returns an integer ranging from 0 through 49. Also, -100+rnd(201) transforms 0.0 to almost 1.0 into -100 through 100 by adding a suitable offset and passing an appropriate limit value. CAUTION: Do not specify (int) Math.random()*limit because this expression always evaluates to 0. The expression first casts random()’s double precision floating-point fractional value (0.0 through 0.99999. . .) to integer 0 by truncating the fractional part, and then multiplies 0 by limit, which results in 0. Table 6–1 also reveals some curiosities beginning with +infinity, -infinity, +0.0, -0.0, and NaN (Not a Number). Java’s floating-point calculations are capable of returning +infinity, -infinity, +0.0, -0.0, and NaN because Java largely conforms to IEEE 754 (http://en.wikipedia.org/wiki/IEEE_754), a standard for floating-point calculations. The following are the circumstances under which these special values arise:  +infinity returns from attempting to divide a positive number by 0.0. For example, System.out.println(1.0/0.0); outputs Infinity.  -infinity returns from attempting to divide a negative number by 0.0. For example, System.out.println(-1.0/0.0); outputs -Infinity. CHAPTER 6: Exploring the Basic APIs Part 1 232  NaN returns from attempting to divide 0.0 by 0.0, attempting to calculate the square root of a negative number, and attempting other strange operations. For example, System.out.println(0.0/0.0); and System.out.println(Math.sqrt(-1.0)); each output NaN.  +0.0 results from attempting to divide a positive number by +infinity. For example, System.out.println(1.0/(1.0/0.0)); outputs +0.0.  -0.0 results from attempting to divide a negative number by +infinity. For example, System.out.println(-1.0/(1.0/0.0)); outputs -0.0. Once an operation yields +infinity, -infinity, or NaN, the rest of the expression usually equals that special value. For example, System.out.println(1.0/0.0*20.0); outputs Infinity. Also, an expression that first yields +infinity or -infinity might devolve into NaN. For example, 1.0/0.0*0.0 yields +infinity (1.0/0.0) and then NaN (+infinity*0.0). Another curiosity is Integer.MAX_VALUE, Integer.MIN_VALUE, Long.MAX_VALUE, and Long.MIN_VALUE. Each of these items is a primitive wrapper class constant that identifies the maximum or minimum value that can be represented by the class’s associated primitive type. Finally, you might wonder why the abs(), max(), and min() overloaded methods do not include byte and short versions, as in byte abs(byte b) and short abs(short s). There is no need for these methods because the limited ranges of bytes and short integers make them unsuitable in calculations. If you need such a method, check out Listing 6–2. Listing 6–2. Overloaded byte abs(byte b) and short abs(short s) methods public static byte abs(byte b) { return (b < 0) ? (byte) -b : b; } public static short abs(short s) { return (s < 0) ? (short) -s : s; } public static void main(String[] args) { byte b = -2; System.out.println(abs(b)); // Output: 2 short s = -3; System.out.println(abs(s)); // Output: 3 } The (byte) and (short) casts are necessary because -b converts b’s value from a byte to an int, and -s converts s’s value from a short to an int. In contrast, these casts are not needed with (b < 0) and (s < 0), which automatically cast b’s and s’s values to an int before comparing them with int-based 0. CHAPTER 6: Exploring the Basic APIs Part 1 233 TIP: Their absence from Math suggests that byte and short are not very useful in method declarations. However, these types are useful when declaring arrays whose elements store small values (such as a binary file’s byte values). If you declared an array of int or long to store such values, you would end up wasting heap space (and might even run out of memory). While searching through the Java documentation for the java.lang package, you will probably encounter a class named StrictMath. Apart from a longer name, this class appears to be identical to Math. The differences between these classes can be summed up as follows:  StrictMath’s methods return exactly the same results on all platforms. In contrast, some of Math’s methods might return values that vary ever so slightly from platform to platform.  Because StrictMath cannot utilize platform-specific features such as an extended-precision math coprocessor, an implementation of StrictMath might be less efficient than an implementation of Math. For the most part, Math’s methods call their StrictMath counterparts. Two exceptions are toDegrees() and toRadians(). Although these methods have identical code bodies in both classes, StrictMath’s implementations include reserved word strictfp in the method headers: public static strictfp double toDegrees(double angrad) public static strictfp double toRadians(double angdeg) Wikipedia’s “strictfp” entry (http://en.wikipedia.org/wiki/Strictfp) mentions that strictfp restricts floating-point calculations to ensure portability. This reserved word accomplishes portability in the context of intermediate floating-point representations and overflows/underflows (generating a value too large or small to fit a representation). NOTE: The previously cited “strictfp” article states that Math contains public static strictfp double abs(double); and other strictfp methods. If you check out this class’s source code under Java version 6 update 16, you will not find strictfp anywhere in the source code. However, many Math methods (such as sin()) call their StrictMath counterparts, which are implemented in a platform-specific library, and the library’s method implementations are strict. Without strictfp, an intermediate calculation is not limited to the IEEE 754 32-bit and 64-bit floating-point representations that Java supports. Instead, the calculation can take advantage of a larger representation (perhaps 128 bits) on a platform that supports this representation. An intermediate calculation that overflows/underflows when its value is represented in 32/64 bits might not overflow/underflow when its value is represented in more bits. CHAPTER 6: Exploring the Basic APIs Part 1 234 Because of this discrepancy, portability is compromised. strictfp levels the playing field by requiring all platforms to use 32/64 bits for intermediate calculations. When applied to a method, strictfp ensures that all floating-point calculations performed in that method are in strict compliance. However, strictfp can be used in a class header declaration (as in public strictfp class FourierTransform) to ensure that all floating-point calculations performed in that class are strict. NOTE: Math and StrictMath are declared final so that they cannot be extended. Also, they declare private empty noargument constructors so that they cannot be instantiated. Math and StrictMath are examples of utility classes because they exist as placeholders for utility constants and utility (static) methods. BigDecimal In Chapter 2, I introduced a CheckingAccount class with a balance field. I declared this field to be of type int, and included a comment stating that balance represents the number of dollars that can be withdrawn. Alternatively, I could have stated that balance represents the number of pennies that can be withdrawn. Perhaps you are wondering why I did not declare balance to be of type double or float. That way, balance could store values such as 18.26 (18 dollars in the whole number part and 26 pennies in the fraction part). I did not declare balance to be a double or float for the following reasons:  Not all floating-point values that can represent monetary amounts (dollars and cents) can be stored exactly in memory. For example, 0.1 (which you might use to represent 10 cents), has no exact storage representation. If you executed double total = 0.1; for (int i = 0; i < 50; i++) total += 0.1; System.out.println(total);, you would observe 5.099999999999998 instead of the correct 5.1 as the output.  The result of each floating-point calculation needs to be rounded to the nearest cent. Failure to do so introduces tiny errors that can cause the final result to differ from the correct result. Although Math supplies a pair of round() methods that you might consider using to round a calculation to the nearest cent, these methods round to the nearest integer (dollar). Listing 6–3’s InvoiceCalc application demonstrates both problems. However, the first problem is not serious because it contributes very little to the inaccuracy. The more serious problem occurs from failing to round to the nearest cent after performing a calculation. This book was purchased by harshamv@variable3.comCHAPTER 6: Exploring the Basic APIs Part 1 235 Listing 6–3. Floating-point-based invoice calculations leading to confusing results import java.text.NumberFormat; class InvoiceCalc { final static double DISCOUNT_PERCENT = 0.1; // 10% final static double TAX_PERCENT = 0.05; // 5% public static void main(String[] args) { double invoiceSubtotal = 285.36; double discount = invoiceSubtotal*DISCOUNT_PERCENT; double subtotalBeforeTax = invoiceSubtotal-discount; double salesTax = subtotalBeforeTax*TAX_PERCENT; double invoiceTotal = subtotalBeforeTax+salesTax; NumberFormat currencyFormat = NumberFormat.getCurrencyInstance(); System.out.println("Subtotal: " + currencyFormat.format(invoiceSubtotal)); System.out.println("Discount: " + currencyFormat.format(discount)); System.out.println("SubTotal after discount: " + currencyFormat.format(subtotalBeforeTax)); System.out.println("Sales Tax: " + currencyFormat.format(salesTax)); System.out.println("Total: " + currencyFormat.format(invoiceTotal)); } } Listing 6–3 relies on the NumberFormat class (located in the java.text) package and its format() method to format a double precision floating-point value into a currency—I will discuss NumberFormat in Chapter 9. When you run InvoiceCalc, you will discover the following output: Subtotal: $285.36 Discount: $28.54 SubTotal after discount: $256.82 Sales Tax: $12.84 Total: $269.67 This output reveals the correct subtotal, discount, subtotal after discount, and sales tax. In contrast, it incorrectly reveals 269.67 instead of 269.66 as the final total. The customer will not appreciate paying an extra penny, even though 269.67 is the correct value according to the floating-point calculations: Subtotal: 285.36 Discount: 28.536 SubTotal after discount: 256.824 Sales Tax: 12.8412 Total: 269.6652 The problem arises from not rounding the result of each calculation to the nearest cent before performing the next calculation. As a result, the 0.024 in 256.824 and 0.0012 in 12.84 contribute to the final value, causing NumberFormat’s format() method to round this value to 269.67. Java provides a solution to both problems in the form of a java.math.BigDecimal class. This immutable class (a BigDecimal instance cannot be modified) represents a signed decimal number (such as 23.653) of arbitrary precision (number of digits) with an associated scale (an integer that specifies the number of digits after the decimal point). CHAPTER 6: Exploring the Basic APIs Part 1 236 BigDecimal declares three convenience constants: ONE, TEN, and ZERO. Each constant is the BigDecimal equivalent of 1, 10, and 0 with a zero scale. CAUTION: BigDecimal declares several ROUND_-prefixed constants. These constants are largely obsolete and should be avoided, along with the public BigDecimal divide(BigDecimal divisor, int scale, int roundingMode) and public BigDecimal setScale(int newScale, int roundingMode) methods, which are still present so that dependent legacy code continues to compile. BigDecimal also declares a variety of useful constructors and methods. A few of these constructors and methods are described in Table 6–2. Table 6–2. BigDecimal Constructors and Methods Method Description BigDecimal(int val) Initialize the BigDecimal instance to val’s digits. Set the scale to 0. BigDecimal(String val) Initialize the BigDecimal instance to the decimal equivalent of val. Set the scale to the number of digits after the decimal point, or 0 if no decimal point is specified. This constructor throws java.lang.NullPointerException when val is null, and java.lang.NumberFormatException when val’s string representation is invalid (contains letters, for example). BigDecimal abs() Return a new BigDecimal instance that contains the absolute value of the current instance’s value. The resulting scale is the same as the current instance’s scale. BigDecimal add(BigDecimal augend) Return a new BigDecimal instance that contains the sum of the current value and the argument value. The resulting scale is the maximum of the current and argument scales. This method throws NullPointerException when augend is null. BigDecimal divide(BigDecimal divisor) Return a new BigDecimal instance that contains the quotient of the current value divided by the argument value. The resulting scale is the difference of the current and argument scales. It might be adjusted when the result requires more digits. This method throws NullPointerException when divisor is null, or java.lang.ArithmeticException when divisor represents 0 or the result cannot be represented exactly. BigDecimal max(BigDecimal val) Return either this or val, whichever BigDecimal instance contains the larger value. This method throws NullPointerException when val is null. CHAPTER 6: Exploring the Basic APIs Part 1 237 Method Description BigDecimal min(BigDecimal val) Return either this or val, whichever BigDecimal instance contains the smaller value. This method throws NullPointerException when val is null. BigDecimal multiply(BigDecimal multiplicand) Return a new BigDecimal instance that contains the product of the current value and the argument value. The resulting scale is the sum of the current and argument scales. This method throws NullPointerException when multiplicand is null. BigDecimal negate() Return a new BigDecimal instance that contains the negative of the current value. The resulting scale is the same as the current scale. int precision() Return the precision of the current BigDecimal instance. BigDecimal remainder(BigDecimal divisor) Return a new BigDecimal instance that contains the remainder of the current value divided by the argument value. The resulting scale is the difference of the current scale and the argument scale. It might be adjusted when the result requires more digits. This method throws NullPointerException when divisor is null, or ArithmeticException when divisor represents 0. int scale() Return the scale of the current BigDecimal instance. BigDecimal setScale(int newScale, RoundingMode roundingMode) Return a new BigDecimal instance with the specified scale and rounding mode. If the new scale is greater than the old scale, additional zeros are added to the unscaled value. In this case no rounding is necessary. If the new scale is smaller than the old scale, trailing digits are removed. If these trailing digits are not zero, the remaining unscaled value has to be rounded. For this rounding operation, the specified rounding mode is used. This method throws NullPointerException when roundingMode is null, and ArithmeticException when roundingMode is set to RoundingMode.ROUND_UNNECESSARY but rounding is necessary based on the current scale. BigDecimal subtract(BigDecimal subtrahend) Return a new BigDecimal instance that contains the current value minus the argument value. The resulting scale is the maximum of the current and argument scales. This method throws NullPointerException when subtrahend is null. String toString() Return a string representation of this BigDecimal. Scientific notation is used when necessary. Table 6–2 refers to RoundingMode, which is an enum containing various rounding mode constants. These constants are described in Table 6–3. CHAPTER 6: Exploring the Basic APIs Part 1 238 Table 6–3. RoundingMode Constants Constant Description CEILING Round toward positive infinity. DOWN Round toward zero. FLOOR Round toward negative infinity. HALF_DOWN Round toward the “nearest neighbor” unless both neighbors are equidistant, in which case round down. HALF_EVEN Round toward the “nearest neighbor” unless both neighbors are equidistant, in which case round toward the even neighbor. HALF_UP Round toward “nearest neighbor” unless both neighbors are equidistant, in which case round up. (This is the rounding mode commonly taught at school.) UNNECESSARY Rounding is not necessary because the requested operation produces the exact result. UP Positive values are rounded toward positive infinity and negative values are rounded toward negative infinity. The best way to get comfortable with BigDecimal is to try it out. Listing 6–4 uses this class to correctly perform the invoice calculations that were presented in Listing 6–3. Listing 6–4. BigDecimal-based invoice calculations not leading to confusing results class InvoiceCalc { public static void main(String[] args) { BigDecimal invoiceSubtotal = new BigDecimal("285.36"); BigDecimal discountPercent = new BigDecimal("0.10"); BigDecimal discount = invoiceSubtotal.multiply(discountPercent); discount = discount.setScale(2, RoundingMode.HALF_UP); BigDecimal subtotalBeforeTax = invoiceSubtotal.subtract(discount); subtotalBeforeTax = subtotalBeforeTax.setScale(2, RoundingMode.HALF_UP); BigDecimal salesTaxPercent = new BigDecimal("0.05"); BigDecimal salesTax = subtotalBeforeTax.multiply(salesTaxPercent); salesTax = salesTax.setScale(2, RoundingMode.HALF_UP); BigDecimal invoiceTotal = subtotalBeforeTax.add(salesTax); invoiceTotal = invoiceTotal.setScale(2, RoundingMode.HALF_UP); System.out.println("Subtotal: " + invoiceSubtotal); System.out.println("Discount: " + discount); System.out.println("SubTotal after discount: " + subtotalBeforeTax); System.out.println("Sales Tax: " + salesTax); System.out.println("Total: " + invoiceTotal); } } CHAPTER 6: Exploring the Basic APIs Part 1 239 Listing 6–4’s main() method first creates BigDecimal objects invoiceSubtotal and discountPercent that are initialized to 285.36 and 0.10, respectively. It multiplies invoiceSubtotal by discountPercent and assigns the BigDecimal result to discount. At this point, discount contains 28.5360. Apart from the trailing zero, this value is the same as that generated by invoiceSubtotal*DISCOUNT_PERCENT in Listing 6–3. The value that should be stored in discount is 28.54. To correct this problem before performing another calculation, main() calls discount’s setScale() method with these arguments:  2: Two digits after the decimal point  RoundingMode.HALF_UP: The conventional approach to rounding After setting the scale and proper rounding mode, main() subtracts discount from invoiceSubtotal, and assigns the resulting BigDecimal instance to subtotalBeforeTax. main() calls setScale() on subtotalBeforeTax to properly round its value before moving on to the next calculation. main() next creates a BigDecimal object named salesTaxPercent that is initialized to 0.05. It then multiplies subtotalBeforeTax by salesTaxPercent, assigning the result to salesTax, and calls setScale() on this BigDecimal object to properly round its value. Moving on, main() adds salesTax to subtotalBeforeTax, saving the result in invoiceTotal, and rounds the result via setScale(). The values in these objects are sent to the standard output device via System.out.println(), which calls their toString() methods to return string representations of the BigDecimal values. When you run this new version of InvoiceCalc, you will discover the following output: Subtotal: 285.36 Discount: 28.54 SubTotal after discount: 256.82 Sales Tax: 12.84 Total: 269.66 CAUTION: BigDecimal declares a public BigDecimal(double val) constructor that you should avoid using if at all possible. This constructor initializes the BigDecimal instance to the value stored in val, making it possible for this instance to reflect an invalid representation when the double cannot be stored exactly. For example, BigDecimal(0.1) results in 0.1000000000000000055511151231257827021181583404541015625 being stored in the instance. In contrast, BigDecimal("0.1") stores 0.1 exactly. BigInteger BigDecimal stores a signed decimal number as an unscaled value with a 32-bit integer scale. The unscaled value is stored in an instance of the java.math.BigInteger class. BigInteger is an immutable class that represents a signed integer of arbitrary precision. It stores its value in two’s complement format (all bits are flipped—1s to 0s and 0s to CHAPTER 6: Exploring the Basic APIs Part 1 240 1s—and 1 is added to the result to be compatible with the two’s complement format used by Java’s byte integer, short integer, integer, and long integer types). NOTE: Check out Wikipedia’s “Two’s complement” entry (http://en.wikipedia.org/wiki/Two%27s_complement) to learn more about two’s complement. BigInteger declares three convenience constants: ONE, TEN, and ZERO. Each constant is the BigInteger equivalent of 1, 10, and 0. BigInteger also declares a variety of useful constructors and methods. A few of these constructors and methods are described in Table 6–4. Table 6–4. BigInteger Constructors and Methods Method Description BigInteger(byte[] val) Initialize the BigInteger instance to the integer that is stored in the val array, with val[0] storing the integer’s most significant (leftmost) eight bits. This constructor throws NullPointerException when val is null, and NumberFormatException when val.length equals 0. BigInteger(String val) Initialize the BigInteger instance to the integer equivalent of val. This constructor throws NullPointerException when val is null, and NumberFormatException when val’s string representation is invalid (contains letters, for example). BigInteger abs() Return a new BigInteger instance that contains the absolute value of the current instance’s value. BigInteger add(BigInteger augend) Return a new BigInteger instance that contains the sum of the current value and the argument value. This method throws NullPointerException when augend is null. BigInteger divide(BigInteger divisor) Return a new BigInteger instance that contains the quotient of the current value divided by the argument value. This method throws NullPointerException when divisor is null, and ArithmeticException when divisor represents 0 or the result cannot be represented exactly. BigInteger max(BigInteger val) Return either this or val, whichever BigInteger instance contains the larger value. This method throws NullPointerException when val is null. BigInteger min(BigInteger val) Return either this or val, whichever BigInteger instance contains the smaller value. This method throws NullPointerException when val is null. CHAPTER 6: Exploring the Basic APIs Part 1 241 Method Description BigInteger multiply(BigInteger multiplicand) Return a new BigInteger instance that contains the product of the current value and the argument value. This method throws NullPointerException when multiplicand is null. BigInteger negate() Return a new BigInteger instance that contains the negative of the current value. BigInteger remainder(BigInteger divisor) Return a new BigInteger instance that contains the remainder of the current value divided by the argument value. This method throws NullPointerException when divisor is null, and ArithmeticException when divisor represents 0. BigInteger subtract(BigInteger subtrahend) Return a new BigInteger instance that contains the current value minus the argument value. This method throws NullPointerException when subtrahend is null. String toString() Return a string representation of this BigInteger. The best way to get comfortable with BigInteger is to try it out. Listing 6–5 uses this class in a factorial() method comparison context. Listing 6–5. Comparing factorial() methods class FactComp { public static void main(String[] args) { System.out.println(factorial(12)); System.out.println(); System.out.println(factorial(20L)); System.out.println(); System.out.println(factorial(170.0)); System.out.println(); System.out.println(factorial(new BigInteger("170"))); System.out.println(); System.out.println(factorial(25.0)); System.out.println(); System.out.println(factorial(new BigInteger("25"))); } public static int factorial(int n) { if (n == 0) return 1; else return n*factorial(n-1); } public static long factorial(long n) { if (n == 0) return 1; else return n*factorial(n-1); CHAPTER 6: Exploring the Basic APIs Part 1 242 } public static double factorial(double n) { if (n == 1.0) return 1.0; else return n*factorial(n-1); } public static BigInteger factorial(BigInteger n) { if (n.equals(BigInteger.ZERO)) return BigInteger.ONE; else return n.multiply(factorial(n.subtract(BigInteger.ONE))); } } Listing 6–5 compares four versions of the recursive factorial() method. This comparison reveals the largest argument that can be passed to each of the first three methods before the returned factorial value becomes meaningless, because of limits on the range of values that can be accurately represented by the numeric type. The first version is based on int and has a useful argument range of 0 through 12. Passing any argument greater than 12 results in a factorial that cannot be represented accurately as an int. You can increase the useful range of factorial(), but not by much, by changing the parameter and return types to long. After making these changes, you will discover that the upper limit of the useful range is 20. To further increase the useful range, you might create a version of factorial() whose parameter and return types are double. This is possible because whole numbers can be represented exactly as doubles. However, the largest useful argument that can be passed is 170.0. Anything higher than this value results in factorial() returning +infinity. It is possible that you might need to calculate a higher factorial value, perhaps in the context of calculating a statistics problem involving combinations or permutations. The only way to accurately calculate this value is to use a version of factorial() based on BigInteger. When you run the previous application, it generates the following output: 479001600 2432902008176640000 7.257415615307994E306 7257415615307998967396728211129263114716991681296451376543577798900561843401706157852350 7492426174595114909912378385207766660225654427530253289007732075109024004302800582956039 6661259965825710439855829425756896631343961226257109494680671120556888045719334021266145 2800000000000000000000000000000000000000000 1.5511210043330986E25 15511210043330985984000000 CHAPTER 6: Exploring the Basic APIs Part 1 243 The first three values represent the highest factorials that can be returned by the int- based, long-based, and double-based factorial() methods. The fourth value represents the BigInteger equivalent of the highest double factorial. Notice that the double method fails to accurately represent 170! (! is the math symbol for factorial). Its precision is simply too small. Although the method attempts to round the smallest digit, rounding does not always work—the number ends in 7994 instead of 7998. Rounding is only accurate up to argument 25.0, as the last two output lines reveal. NOTE: RSA encryption, BigDecimal, and factorial are practical examples of BigInteger’s usefulness. However, you can also use BigInteger in unusual ways. For example, my February 2006 JavaWorld article titled “Travel Through Time with Java” (http://www.javaworld.com/javaworld/jw-02-2006/jw-0213-funandgames.html), a part of my Java Fun and Games series, used BigInteger to store an image as a very large integer. The idea was to experiment with BigInteger methods to look for images of people and places that existed in the past, will exist in the future, or might never exist. If this craziness appeals to you, check out my article. Package Information The java.lang.Package class provides access to information about a package (see Chapter 4 for an introduction to packages). This information includes version information about the implementation and specification of a Java package, the name of the package, and an indication of whether or not the package has been sealed (all classes that are part of the package are archived in the same JAR file). NOTE: Chapter 1 introduces JAR files. Table 6–5 describes some of Package’s methods. Table 6–5. Package Methods Method Description String getImplementationTitle() Return the title of this package’s implementation, which might be null. The format of the title is unspecified. String getImplementationVendor() Return the name of the vendor or organization that provides this package’s implementation. This name might be null. The format of the name is unspecified. String getImplementationVersion() Return the version number of this package’s implementation, which might be null. This version string must be a sequence of positive decimal integers separated by periods and might have leading zeros. CHAPTER 6: Exploring the Basic APIs Part 1 244 Method Description String getName() Return the name of this package in standard dot notation; for example, java.lang. static Package getPackage(String packageName) Return the Package object that is associated with the package identified as packageName, or null when the package identified as packageName cannot be found. This method throws NullPointerException when packageName is null. static Package[] getPackages() Return an array of all Package objects that are accessible to this method’s caller. String getSpecificationTitle() Return the title of this package’s specification, which might be null. The format of the title is unspecified. String getSpecificationVendor() Return the name of the vendor or organization that provides the specification that is implemented by this package. This name might be null. The format of the name is unspecified. String getSpecificationVersion() Return the version number of the specification of this package’s implementation, which might be null. This version string must be a sequence of positive decimal integers separated by periods, and might have leading zeros. boolean isCompatibleWith(String desired) Check this package to determine if it is compatible with the specified version string, by comparing this package’s specification version with the desired version. Return true when this package’s specification version number is greater than or equal to the desired version number (this package is compatible); otherwise, return false. This method throws NullPointerException when desired is null, and NumberFormatException when this package’s version number or the desired version number is not in dotted form. boolean isSealed() Return true when this package has been sealed; otherwise, return false. I have created a PackageInfo application that demonstrates most of Table 6–5’s Package methods. Listing 6–6 presents this application’s source code. Listing 6–6. Obtaining information about a package public class PackageInfo { public static void main(String[] args) { if (args.length == 0) { System.err.println("usage: java PackageInfo packageName [version]"); return; } This book was purchased by harshamv@variable3.comCHAPTER 6: Exploring the Basic APIs Part 1 245 Package pkg = Package.getPackage(args[0]); if (pkg == null) { System.err.println(args[0] + " not found"); return; } System.out.println("Name: " + pkg.getName()); System.out.println("Implementation title: " + pkg.getImplementationTitle()); System.out.println("Implementation vendor: " + pkg.getImplementationVendor()); System.out.println("Implementation version: " + pkg.getImplementationVersion()); System.out.println("Specification title: " + pkg.getSpecificationTitle()); System.out.println("Specification vendor: " + pkg.getSpecificationVendor()); System.out.println("Specification version: " + pkg.getSpecificationVersion()); System.out.println("Sealed: " + pkg.isSealed()); if (args.length > 1) System.out.println("Compatible with " + args[1] + ": " + pkg.isCompatibleWith(args[1])); } } To use this application, specify at least a package name on the command line. For example, java PackageInfo java.lang returns the following output under Java version 6: Name: java.lang Implementation title: Java Runtime Environment Implementation vendor: Sun Microsystems, Inc. Implementation version: 1.6.0_16 Specification title: Java Platform API Specification Specification vendor: Sun Microsystems, Inc. Specification version: 1.6 Sealed: false PackageInfo also lets you determine if the package’s specification is compatible with a specific version number. A package is compatible with its predecessors. For example, java PackageInfo java.lang 1.6 outputs Compatible with 1.6: true, whereas java PackageInfo java.lang 1.8 outputs Compatible with 1.8: false. You can also use PackageInfo with your own packages, which you learned to create in Chapter 4. For example, that chapter presented a logging package. Copy PackageInfo.class into the directory containing the logging package directory (which contains the compiled classfiles), and execute java PackageInfo logging. PackageInfo responds by displaying the following output: logging not found This error message is presented because getPackage() requires at least one classfile to be loaded from the package before it returns a Package object describing that package. CHAPTER 6: Exploring the Basic APIs Part 1 246 The only way to eliminate the previous error message is to load a class from the package. Accomplish this task by merging Listing 6–7 into Listing 6–6. Listing 6–7. Dynamically loading a class from a classfile if (args.length == 3) try { Class.forName(args[2]); } catch (ClassNotFoundException cnfe) { System.err.println("cannot load " + args[2]); return; } This code fragment, which must precede Package pkg = Package.getPackage(args[0]);, loads the classfile named by the revised PackageInfo application’s third command-line argument. Run the new PackageInfo application via java PackageInfo logging 1.5 logging.File and you will observe the following output—this command line identifies logging’s File class as the class to load: Name: logging Implementation title: null Implementation vendor: null Implementation version: null Specification title: null Specification vendor: null Specification version: null Sealed: false Exception in thread "main" java.lang.NumberFormatException: Empty version string at java.lang.Package.isCompatibleWith(Unknown Source) at PackageInfo.main(PackageInfo.java:43) It is not surprising to see all of these null values because no package information has been added to the logging package. Also, NumberFormatException is thrown from isCompatibleWith() because the logging package does not contain a specification version number in dotted form (it is null). Perhaps the simplest way to place package information into the logging package is to create a logging.jar file in a similar manner to the example shown in Chapter 4. But first, you must create a small text file that contains the package information. You can choose any name for the file. Listing 6–8 reveals my choice of manifest.mf. Listing 6–8. manifest.mf containing the package information Implementation-Title: Logging Implementation Implementation-Vendor: Jeff Friesen Implementation-Version: 1.0a Specification-Title: Logging Specification Specification-Vendor: Jeff "JavaJeff" Friesen Specification-Version: 1.0 Sealed: true CHAPTER 6: Exploring the Basic APIs Part 1 247 NOTE: Make sure to press the Return/Enter key at the end of the final line (Sealed: true). Otherwise, you will probably observe Sealed: false in the output because this entry will not be stored in the logging package by the JDK’s jar tool—jar is a bit quirky. Execute the following command line to create a JAR file that includes logging and its files, and whose manifest, a special file named MANIFEST.MF that stores information about the contents of a JAR file, contains the contents of Listing 6–8: jar cfm logging.jar manifest.mf logging This command line creates a JAR file named logging.jar (via the c [create] and f [file] options). It also merges the contents of manifest.mf (via the m [manifest] option) into MANIFEST.MF, which is stored in the package’s META-INF directory. NOTE: To learn more about a JAR file’s manifest, read the “JAR Manifest” section of the JDK documentation’s “JAR File Specification” page (http://java.sun.com/javase/6/docs/technotes/guides/jar/jar.html#JAR%20M anifest). Assuming that the jar tool presents no error messages, execute the following Windows- oriented command line (or a command line suitable for your platform) to run PackageInfo and extract the package information from the logging package: java -cp logging.jar;. PackageInfo logging 1.0 logging.File This time, you should see the following output: Name: logging Implementation title: Logging Implementation Implementation vendor: Jeff Friesen Implementation version: 1.0a Specification title: Logging Specification Specification vendor: Jeff "JavaJeff" Friesen Specification version: 1.0 Sealed: true Compatible with 1.0: true Primitive Wrapper Classes The java.lang package includes Boolean, Byte, Character, Double, Float, Integer, Long, and Short. These classes are known as primitive wrapper classes because their instances wrap themselves around values of primitive types. NOTE: The primitive wrapper classes are also known as value classes. CHAPTER 6: Exploring the Basic APIs Part 1 248 Java provides these eight primitive wrapper classes for two reasons:  The collections framework (discussed Chapter 8) provides lists, sets, and maps that can only store objects; they cannot store primitive values. You store a primitive value in a primitive wrapper class instance and store the instance in the collection.  These classes provide a good place to associate useful constants (such as MAX_VALUE and MIN_VALUE) and class methods (such as Integer’s parseInt() methods and Character’s isDigit(), isLetter(), and toUpperCase() methods) with the primitive types. This section introduces you to each of these primitive wrapper classes and a class named Number. Boolean Boolean is the smallest of the primitive wrapper classes. This class declares three constants, including TRUE and FALSE, which denote precreated Boolean objects. Boolean also declares a pair of constructors for initializing a Boolean object:  Boolean(boolean value) initializes the Boolean object to value.  Boolean(String s) converts s’s text to a true or false value and stores this value in the Boolean object. The second constructor compares s’s value with true. Because the comparison is case- insensitive, any combination of these four letters (such as true, TRUE, or tRue) results in true being stored in the object. Otherwise, the constructor stores false in the object. Boolean’s constructors are complemented by boolean booleanValue(), which returns the wrapped Boolean value. Boolean also declares or overrides the following methods:  int compareTo(Boolean b) compares the current Boolean object with b to determine their relative order. The method returns 0 when the current object contains the same Boolean value as b, a positive value when the current object contains true and b contains false, and a negative value when the current object contains false and b contains true.  boolean equals(Object o) compares the current Boolean object with o and returns true when o is not null, o is of type Boolean, and both objects contain the same Boolean value.  static boolean getBoolean(String name) returns true when a system property (discussed in Chapter 7) identified by name exists and is equal to true. CHAPTER 6: Exploring the Basic APIs Part 1 249  int hashCode() returns a suitable hash code that allows Boolean objects to be used with hash-based collections (discussed in Chapter 8).  static boolean parseBoolean(String s) parses s, returning true if s equals "true", "TRUE", "True", or any other combination of these letters. Otherwise, this method returns false. (Parsing breaks a sequence of characters into meaningful components, known as tokens.)  String toString() returns "true" when the current Boolean instance contains true; otherwise, this method returns "false".  static String toString(boolean b) returns "true" when b contains true; otherwise, this method returns "false".  static Boolean valueOf(boolean b) returns TRUE when b contains true or FALSE when b contains false.  static Boolean valueOf(String s) returns TRUE when s equals "true", "TRUE", "True", or any other combination of these letters. Otherwise, this method returns FALSE. CAUTION: Newcomers to the Boolean class often think that getBoolean() returns a Boolean object’s true/false value. However, getBoolean() returns the value of a Boolean- based system property—I discuss system properties in Chapter 7. If you need to return a Boolean object’s true/false value, use the booleanValue() method instead. It is often better to use TRUE and FALSE than to create Boolean objects. For example, suppose you need a method that returns a Boolean object containing true when the method’s double argument is negative, or false when this argument is zero or positive. You might declare your method like the isNegative() method shown in Listing 6–9. Listing 6–9. An isNegative() method with unnecessary Boolean object creation public Boolean isNegative(double d) { return new Boolean(d < 0); } Although this method is concise, it unnecessarily creates a Boolean object. When the method is called frequently, many Boolean objects are created that consume heap space. When heap space runs low, the garbage collector runs and slows down the application, which impacts performance. Listing 6–10 reveals a better way to code isNegative(). Listing 6–10. A refactored isNegative() method not creating Boolean objects public Boolean isNegative(double d) { return (d < 0) ? Boolean.TRUE : Boolean.FALSE; } CHAPTER 6: Exploring the Basic APIs Part 1 250 This method avoids creating Boolean objects by returning either the precreated TRUE or FALSE object. TIP: You should strive to create as few objects as possible. Not only will your applications have smaller memory footprints, they will perform better because the garbage collector will not be required to run as often. Character Character is the largest of the primitive wrapper classes, containing many constants, a constructor, many methods, and a pair of nested classes (Subset and UnicodeBlock). NOTE: Character’s complexity derives from Java’s support for Unicode (http://en.wikipedia.org/wiki/Unicode). For brevity, I ignore much of Character’s Unicode-related complexity, which is beyond the scope of this chapter. Character declares a single Character(char value) constructor, which you use to initialize a Character object to value. This constructor is complemented by char charValue(), which returns the wrapped character value. When you start writing applications, you might codify expressions such as ch >= '0' && ch <= '9' (test ch to see if it contains a digit) and ch >= 'A' && ch <= 'Z' (test ch to see if it contains an uppercase letter). You should avoid doing so for three reasons:  It is too easy to introduce a bug into the expression. For example, ch > '0' && ch <= '9' introduces a subtle bug that does not include '0' in the comparison.  The expressions are not very descriptive of what they are testing.  The expressions are biased toward Latin digits (0–9) and letters (A–Z and a–z). They do not take into account digits and letters that are valid in other languages. For example, '\u0beb' is a character literal representing one of the digits in the Tamil language. Character declares several comparison and conversion utility methods that address these concerns. These methods include the following:  static boolean isDigit(char ch) returns true when ch contains a digit (typically 0 through 9, but also digits in other languages).  static boolean isLetter(char ch) returns true when ch contains a letter (typically A–Z or a–z, but also letters in other languages).  static boolean isLetterOrDigit(char ch) returns true when ch contains a letter or digit (typically A–Z, a–z, or 0–9, but also letters or digits in other languages). CHAPTER 6: Exploring the Basic APIs Part 1 251  static boolean isLowerCase(char ch) returns true when ch contains a lowercase letter.  static boolean isUpperCase(char ch) returns true when ch contains an uppercase letter.  static boolean isWhitespace(char ch) returns true when ch contains a whitespace character (typically a space, a horizontal tab, a carriage return, or a line feed).  static char toLowerCase(char ch) returns the lowercase equivalent of ch’s uppercase letter; otherwise, this method returns ch’s value.  static char toUpperCase(char ch) returns the uppercase equivalent of ch’s lowercase letter; otherwise, this method returns ch’s value. For example, isDigit(ch) is preferable to ch >= '0' && ch <= '9' because it avoids a source of bugs, is more readable, and returns true for non-Latin digits (such as '\u0beb') as well as Latin digits. Float and Double Float and Double store floating-point and double precision floating-point values in Float and Double objects, respectively. These classes declare the following constants:  MAX_VALUE identifies the maximum value that can be represented as a float or double.  MIN_VALUE identifies the minimum value that can be represented as a float or double.  NaN represents 0.0F/0.0F as a float and 0.0/0.0 as a double.  NEGATIVE_INFINITY represents -infinity as a float or double.  POSITIVE_INFINITY represents +infinity as a float or double. Float and Double also declare the following constructors for initializing their objects:  Float(float value) initializes the Float object to value.  Float(double value) initializes the Float object to the float equivalent of value.  Float(String s) converts s’s text to a floating-point value and stores this value in the Float object.  Double(double value) initializes the Double object to value.  Double(String s) converts s’s text to a double precision floating-point value and stores this value in the Double object. CHAPTER 6: Exploring the Basic APIs Part 1 252 Float’s constructors are complemented by float floatValue(), which returns the wrapped floating-point value. Similarly, Double’s constructors are complemented by double doubleValue(), which returns the wrapped double precision floating-point value. Float declares several utility methods in addition to floatValue(). These methods include the following:  static int floatToIntBits(float value) converts value to a 32-bit integer.  static boolean isInfinite(float f) returns true when f’s value is +infinity or -infinity. A related public boolean isInfinite() method returns true when the current Float object’s value is +infinity or - infinity.  static boolean isNaN(float f) returns true when f’s value is NaN. A related public boolean isNaN() method returns true when the current Float object’s value is NaN.  static float parseFloat(String s) parses s, returning the floating- point equivalent of s’s textual representation of a floating-point value or throwing NumberFormatException when this representation is invalid (contains letters, for example). Double declares several utility methods in addition to doubleValue(). These methods include the following:  static long doubleToLongBits(double value) converts value to a long integer.  static boolean isInfinite(double d) returns true when d’s value is +infinity or -infinity. A related boolean isInfinite() method returns true when the current Double object’s value is +infinity or -infinity.  static boolean isNaN(double d) returns true when d’s value is NaN. A related public boolean isNaN() method returns true when the current Double object’s value is NaN.  static double parseDouble(String s) parses s, returning the double precision floating-point equivalent of s’s textual representation of a double precision floating-point value or throwing NumberFormatException when this representation is invalid. The floatToIntBits() and doubleToIntBits() methods are used in implementations of the equals() and hashCode() methods that must take float and double fields into account. floatToIntBits() and doubleToIntBits() allow equals() and hashCode() to respond properly to the following situations: CHAPTER 6: Exploring the Basic APIs Part 1 253  equals() must return true when f1 and f2 contain Float.NaN (or d1 and d2 contain Double.NaN). If equals() was implemented in a manner similar to f1.floatValue() == f2.floatValue() (or d1.doubleValue() == d2.doubleValue()), this method would return false because NaN is not equal to anything, including itself.  equals() must return false when f1 contains +0.0 and f2 contains -0.0 (or vice versa), or d1 contains +0.0 and d2 contains -0.0 (or vice versa). If equals() was implemented in a manner similar to f1.floatValue() == f2.floatValue() (or d1.doubleValue() == d2.doubleValue()), this method would return true because +0.0 == -0.0 returns true. These requirements are needed for hash-based collections (discussed in Chapter 8) to work properly. Listing 6–11 shows how they impact Float’s and Double’s equals() methods. Listing 6–11. Demonstrating Float’s equals() method in a NaN context and Double’s equals() method in a +/-0.0 context public static void main(String[] args) { Float f1 = new Float(Float.NaN); System.out.println(f1.floatValue()); Float f2 = new Float(Float.NaN); System.out.println(f2.floatValue()); System.out.println(f1.equals(f2)); System.out.println(Float.NaN == Float.NaN); System.out.println(); Double d1 = new Double(+0.0); System.out.println(d1.doubleValue()); Double d2 = new Double(-0.0); System.out.println(d2.doubleValue()); System.out.println(d1.equals(d2)); System.out.println(+0.0 == -0.0); } Run this application. The following output proves that Float’s equals() method properly handles NaN and Double’s equals() method properly handles +/-0.0: NaN NaN true false 0.0 -0.0 false true TIP: If you want to test a float or double value for equality with +infinity or -infinity (but not both), do not use isInfinite(). Instead, compare the value with NEGATIVE_INFINITY or POSITIVE_INFINITY via ==. For example, f == Float.NEGATIVE_INFINITY. CHAPTER 6: Exploring the Basic APIs Part 1 254 You will find parseFloat() and parseDouble() useful in many contexts. For example, Listing 6–12 uses parseDouble() to parse command-line arguments into doubles. Listing 6–12. Parsing command-line arguments into double precision floating-point values public static void main(String[] args) { if (args.length != 3) { System.err.println("usage: java Calc value1 op value2"); System.err.println("op is one of +, -, *, or /"); return; } try { double value1 = Double.parseDouble(args[0]); double value2 = Double.parseDouble(args[2]); if (args[1].equals("+")) System.out.println(value1+value2); else if (args[1].equals("-")) System.out.println(value1-value2); else if (args[1].equals("*")) System.out.println(value1*value2); else if (args[1].equals("/")) System.out.println(value1/value2); else System.err.println("invalid operator: " + args[1]); } catch (NumberFormatException nfe) { System.err.println("Bad number format: " + nfe.getMessage()); } } Specify java Calc 10E+3 + 66.0 to try out the Calc application. This application responds by outputting 10066.0. If you specified java Calc 10E+3 + A instead, you would observe Bad number format: For input string: "A" as the output, which is in response to the second parseDouble() method call’s throwing of a NumberFormatException object. Although NumberFormatException describes an unchecked exception, and although unchecked exceptions are often not handled because they represent coding mistakes, NumberFormatException does not fit this pattern in this example. The exception does not arise from a coding mistake; it arises from someone passing an illegal numeric argument to the application, which cannot be avoided through proper coding. This book was purchased by harshamv@variable3.comCHAPTER 6: Exploring the Basic APIs Part 1 255 Integer, Long, Short, and Byte Integer, Long, Short, and Byte store 32-bit, 64-bit, 16–bit, and 8-bit integer values in Integer, Long, Short, and Byte objects, respectively. Each class declares MAX_VALUE and MIN_VALUE constants that identify the maximum and minimum values that can be represented by its associated primitive type. These classes also declare the following constructors for initializing their objects:  Integer(int value) initializes the Integer object to value.  Integer(String s) converts s’s text to a 32-bit integer value and stores this value in the Integer object.  Long(long value) initializes the Long object to value.  Long(String s) converts s’s text to a 64-bit integer value and stores this value in the Long object.  Short(short value) initializes the Short object to value.  Short(String s) converts s’s text to a 16–bit integer value and stores this value in the Short object.  Byte(byte value) initializes the Byte object to value.  Byte(String s) converts s’s text to an 8-bit integer value and stores this value in the Byte object. Integer’s constructors are complemented by int intValue(), Long’s constructors are complemented by long longValue(), Short’s constructors are complemented by short shortValue(), and Byte’s constructors are complemented by byte byteValue(). These methods return wrapped integers. These classes declare various useful integer-oriented methods. For example, Integer declares the following class methods for converting a 32-bit integer to a String according to a specific representation (binary, hexadecimal, octal, and decimal):  static String toBinaryString(int i) returns a String object containing i’s binary representation. For example, Integer.toBinaryString(255) returns a String object containing 11111111.  static String toHexString(int i) returns a String object containing i’s hexadecimal representation. For example, Integer.toHexString(255) returns a String object containing ff.  static String toOctalString(int i) returns a String object containing i’s octal representation. For example, toOctalString(64) returns a String object containing 377. CHAPTER 6: Exploring the Basic APIs Part 1 256  static String toString(int i) returns a String object containing i’s decimal representation. For example, toString(255) returns a String object containing 255. It is often convenient to prepend zeros to a binary string so that you can align multiple binary strings in columns. For example, you might want to create an application that displays the following aligned output: 11110001 + 00000111 -------- 11111000 Unfortunately, toBinaryString() does not let you accomplish this task. For example, Integer.toBinaryString(7) returns a String object containing 111 instead of 00000111. Listing 6–13’s toAlignedBinaryString() method addresses this oversight. Listing 6–13. Aligning binary strings public static void main(String[] args) { System.out.println(toAlignedBinaryString(7, 8)); System.out.println(toAlignedBinaryString(255, 16)); System.out.println(toAlignedBinaryString(255, 7)); } static String toAlignedBinaryString(int i, int numBits) { String result = Integer.toBinaryString(i); if (result.length() > numBits) return null; // cannot fit result into numBits columns int numLeadingZeros = numBits-result.length(); String zerosPrefix = ""; for (int j = 0; j < numLeadingZeros; j++) zerosPrefix += "0"; return zerosPrefix + result; } The toAlignedBinaryString() method takes two arguments: the first argument specifies the 32-bit integer that is to be converted into a binary string, and the second argument specifies the number of bit columns in which to fit the string. After calling toBinaryString() to return i’s equivalent binary string without leading zeros, toAlignedBinaryString() verifies that the string’s digits can fit into the number of bit columns specified by numBits. If they do not fit, this method returns null. (You will learn about length() and other String methods in Chapter 7.) Moving on, toAlignedBinaryString() calculates the number of leading "0"s to prepend to result, and then uses a for loop to create a string of leading zeros. This method ends by returning the leading zeros string prepended to the result string. Although using the compound string concatenation with assignment operator (+=) in a loop to build a string looks okay, it is very inefficient because intermediate String objects are created and thrown away. However, I employed this inefficient code so that I can contrast it with the more efficient code that I present in Chapter 7. CHAPTER 6: Exploring the Basic APIs Part 1 257 When you run this application, it generates the following output: 00000111 0000000011111111 null Number Each of Float, Double, Integer, Long, Short, and Byte provides the other classes’ xValue() methods in addition to its own xValue() method. For example, Float provides doubleValue(), intValue(), longValue(), shortValue(), and byteValue() in addition to floatValue(). All six methods are members of java.lang.Number, which is the abstract superclass of Float, Double, Integer, Long, Short, and Byte—Number’s floatValue(), doubleValue(), intValue(), and longValue() methods are abstract. Number is also the superclass of BigDecimal and BigInteger (and some concurrency-related classes; see Chapter 9). Number exists to simplify iterating over a collection of Number subclass objects. For example, you can declare a variable of List type and initialize it to an instance of ArrayList. You can then store a mixture of Number subclass objects in the collection, and iterate over this collection by calling a subclass method polymorphically. References API Chapter 2 introduced you to garbage collection, where you learned that the garbage collector removes an object from the heap when there are no more references to the object. Chapter 3 introduced you to Object’s finalize() method, where you learned that the garbage collector calls this method before removing an object from the heap. This method gives the object an opportunity to perform cleanup. This section continues from where Chapters 2 and 3 left off by introducing you to Java’s References API. This API makes it possible for an application to interact with the garbage collector in limited ways. The section first acquaints you with some basic terminology. It then introduces you to the API’s Reference and ReferenceQueue classes, followed by the API’s SoftReference, WeakReference, and PhantomReference classes. Basic Terminology When an application runs, its execution reveals a root set of references, a collection of local variables, parameters, class fields, and instance fields that currently exist and that contain (possibly null) references to objects. This root set changes over time as the application runs. For example, parameters disappear after a method returns. CHAPTER 6: Exploring the Basic APIs Part 1 258 Many garbage collectors identify this root set when they run. They use the root set to determine if an object is reachable (referenced, also known as live) or unreachable (not referenced). The garbage collector cannot collect reachable objects. Instead, it can only collect objects that, starting from the root set of references, cannot be reached. NOTE: Reachable objects include objects that are indirectly reachable from root-set variables, which means objects that are reachable through live objects that are directly reachable from those variables. An object that is unreachable by any path from any root-set variable is eligible for garbage collection. Beginning with Java version 1.2, reachable objects were classified as strongly reachable, softly reachable, weakly reachable, and phantom reachable. Unlike strongly reachable objects, softly, weakly, and phantom reachable objects can be garbage collected. The following list describes these four kinds of reachability in terms of reference strength, from strongest to weakest:  An object is strongly reachable when it is reachable by a thread without the thread having to traverse References API objects—the thread follows a strong reference in a root-set variable. A newly created object (such as the object referenced by d in Double d = new Double(1.0);) is strongly reachable by the thread that created it. (I discuss threads in Chapter 7.)  An object is softly reachable when it is not strongly reachable but can be reached by traversing a soft reference (a reference to the object where the reference is stored in a SoftReference object). The strongest reference to this object is a soft reference. When heap memory runs low, the garbage collector typically clears the soft references of the oldest softly reachable objects and removes those objects after finalizing them (by calling finalize()).  An object is weakly reachable when it is not strongly or softly reachable but can be reached by traversing a weak reference (a reference to the object where the reference is stored in a WeakReference object). The strongest reference to this object is a weak reference. The garbage collector clears weak references to weakly reachable objects and throws away these objects (after finalizing them) the next time it runs, even when memory is plentiful.  An object is phantom reachable when it is neither strongly, softly, nor weakly reachable, it has been finalized, and the garbage collector is ready to reclaim its memory. Furthermore, it is referred to by some phantom reference (a reference to the object where the reference is stored in a PhantomReference object). The strongest reference to this object is a phantom reference. CHAPTER 6: Exploring the Basic APIs Part 1 259 NOTE: Apart from the garbage collector being less eager to clean up the softly reachable object, a soft reference is exactly like a weak reference. Also, a weak reference is not strong enough to keep an object in memory. The object whose reference is stored in a SoftReference, WeakReference, or PhantomReference object is known as a referent. Reference and ReferenceQueue The References API consists of five classes located in the java.lang.ref package. Central to this package are Reference and ReferenceQueue. Reference is the abstract superclass of this package’s concrete SoftReference, WeakReference, and PhantomReference subclasses. ReferenceQueue is a concrete class whose instances describe queue data structures. When you associate a ReferenceQueue instance with a Reference subclass object (Reference object, for short), the Reference object is added to the queue when the referent to which its encapsulated reference refers becomes garbage. NOTE: You associate a ReferenceQueue object with a Reference object by passing the ReferenceQueue object to an appropriate Reference subclass constructor. Reference is declared as generic type Reference, where T identifies the referent’s type. This class provides the following methods:  void clear() assigns null to the stored reference; the Reference object on which this method is called is not enqueued (inserted) into its associated reference queue (if there is an associated reference queue). (The garbage collector clears references directly; it does not call clear(). Instead, this method is called by applications.)  boolean enqueue() adds the Reference object on which this method is called to the associated reference queue. This method returns true when this Reference object has become enqueued; otherwise, this method returns false—this Reference object was already enqueued or was not associated with a queue when created. (The garbage collector enqueues Reference objects directly; it does not call enqueue(). Instead, this method is called by applications.)  T get() returns this Reference object’s stored reference. The return value is null when the stored reference has been cleared, either by the application or by the garbage collector. CHAPTER 6: Exploring the Basic APIs Part 1 260  boolean isEnqueued() returns true when this Reference object has been enqueued, either by the application or by the garbage collector. Otherwise, this method returns false—this Reference object was not associated with a queue when created. NOTE: Reference also declares constructors. Because these constructors are package-private, only classes in the java.lang.ref package can subclass Reference. This restriction is necessary because instances of Reference’s subclasses must work closely with the garbage collector. ReferenceQueue is declared as generic type ReferenceQueue, where T identifies the referent’s type. This class declares the following constructor and methods:  ReferenceQueue() initializes a new ReferenceQueue instance.  Reference poll() polls this queue to check for an available Reference object. If one is available, the object is removed from the queue and returned. Otherwise, this method returns immediately with a null value.  Reference remove() removes the next Reference object from the queue and returns this object. This method waits indefinitely for a Reference object to become available, and throws java.lang.InterruptedException when this wait is interrupted.  Reference remove(long timeout) removes the next Reference object from the queue and returns this object. This method waits until a Reference object becomes available or until timeout milliseconds have elapsed—passing 0 to timeout causes the method to wait indefinitely. If timeout’s value expires, the method returns null. This method throws java.lang.IllegalArgumentException when timeout’s value is negative, or InterruptedException when this wait is interrupted. SoftReference The SoftReference class describes a Reference object whose referent is softly reachable. In addition to inheriting Reference’s methods and overriding get(), this generic class provides the following constructors for initializing a SoftReference object:  SoftReference(T r) encapsulates r’s reference. The SoftReference object behaves as a soft reference to r. No ReferenceQueue object is associated with this SoftReference object. CHAPTER 6: Exploring the Basic APIs Part 1 261  SoftReference(T r, ReferenceQueue q) encapsulates r’s reference. The SoftReference object behaves as a soft reference to r. The ReferenceQueue object identified by q is associated with this SoftReference object. Passing null to q indicates a soft reference without a queue. SoftReference is useful for implementing caches, such as a cache of images. An image cache keeps images in memory (because it takes time to load them from disk) and ensures that duplicate (and possibly very large) images are not stored in memory. The image cache contains references to image objects that are already in memory. If these references were strong, the images would remain in memory. You would then need to figure out which images are no longer needed and remove them from memory so that they can be garbage collected. Having to manually remove images duplicates the work of a garbage collector. However, if you wrap the references to the image objects in SoftReference objects, the garbage collector will determine when to remove these objects (typically when heap memory runs low) and perform the removal on your behalf. Listing 6–14 shows how you might use SoftReference to maintain a cache of images. Listing 6–14. Maintaining a cache of images class Image { private byte[] image; private Image(String name) { image = new byte[1024*100]; } static Image getImage(String name) { return new Image(name); } } public class ImageCache { final static int NUM_IMAGES = 200; @SuppressWarnings("unchecked") public static void main(String[] args) { String[] imageNames = new String[NUM_IMAGES]; for (int i = 0; i < imageNames.length; i++) imageNames[i] = new String("image" + i + ".gif"); SoftReference[] cache = new SoftReference[imageNames.length]; for (int i = 0; i < cache.length; i++) cache[i] = new SoftReference(Image.getImage(imageNames[i])); for (int i = 0; i < cache.length; i++) { Image im = cache[i].get(); if (im == null) { CHAPTER 6: Exploring the Basic APIs Part 1 262 System.out.println(imageNames[i] + " not in cache"); im = Image.getImage(imageNames[i]); cache[i] = new SoftReference(im); } System.out.println("Drawing image"); im = null; // Remove strong reference to image. } } } This listing declares an Image class that simulates a loaded image. Each instance is created by calling the getImage() class method, and the instance’s private image array occupies 100KB of memory. The main() method first creates an array of String objects that contain image filenames. The technique employed in creating this array is inefficient. You will discover an efficient alternative in Chapter 7. main() next creates an array of SoftReference objects that serves as a cache for Image objects. This array is initialized to SoftReference objects; each SoftReference object is initialized to an Image object’s reference. main() now enters the application’s main loop. It iterates over the cache, retrieving each Image object or null when the garbage collector has cleared the soft reference to the Image object (so that it can make room in the heap). If the reference assigned to im is not null, the Image object has not been made unreachable and subsequent code can draw the image on the screen. The im = null; assignment removes the strong reference to the Image object from the im root-set variable. NOTE: The im = null; assignment is not necessary in this application because either im is immediately overwritten by get()’s return value in the next loop iteration, or the loop and the application ends. Because im’s value might hang around for a while in a refactored and longer- lived version of this application, and the garbage collector would not be able to remove the associated Image object from the heap because that object would be strongly reachable, I’ve included this assignment to show you how to get rid of im’s value. When the reference assigned to im is null, the Image object has been made unreachable and has probably been removed from the heap. In this case, the Image object must be re-created and stored in a new SoftReference object that is stored in the cache. Here is a small portion of the output that I observed—you may have to adjust the application’s code to observe similar output: image162.gif not in cache Drawing image image163.gif not in cache Drawing image Drawing image CHAPTER 6: Exploring the Basic APIs Part 1 263 Regarding the last line of output, its Drawing image message implies that image164.gif is still in the cache. In other words, the associated Image object is still reachable. NOTE: If you observe an unending repetition of the Drawing image message, perhaps your Java virtual machine’s heap space is larger than the heap space used by my virtual machine when I ran this application on my Windows XP platform. If your virtual machine’s heap space is large enough, soft references will not be cleared and you will end up with an infinite loop of output. To correct this situation, you might want to increase the size of Image’s image array (perhaps from 1024*100 to 1024*500) and (possibly) assign a larger value to NUM_IMAGES (perhaps 500). WeakReference The WeakReference class describes a Reference object whose referent is weakly reachable. In addition to inheriting Reference’s methods, this generic class provides the following constructors for initializing a WeakReference object:  WeakReference(T r) encapsulates r’s reference. The WeakReference object behaves as a weak reference to r. No ReferenceQueue object is associated with this WeakReference object.  WeakReference(T r, ReferenceQueue q) encapsulates r’s reference. The WeakReference object behaves as a weak reference to r. The ReferenceQueue object identified by q is associated with this WeakReference object. Passing null to q indicates a weak reference without a queue. WeakReference is useful for preventing memory leaks related to hashmaps. A memory leak occurs when you keep adding objects to a hashmap and never remove them. The objects remain in memory because the hashmap stores strong references to them. Ideally, the objects should only remain in memory when they are strongly referenced from elsewhere in the application. When an object’s last strong reference (apart from hashmap strong references) disappears, the object should be garbage collected. This situation can be remedied by storing weak references to hashmap entries so they are discarded when no strong references to their keys exist. Java’s WeakHashmap class (discussed in Chapter 8) accomplishes this task. PhantomReference The PhantomReference class describes a Reference object whose referent is phantom reachable. In addition to inheriting Reference’s methods and overriding get(), this generic class provides a single constructor for initializing a PhantomReference object: CHAPTER 6: Exploring the Basic APIs Part 1 264  PhantomReference(T r, ReferenceQueue q) encapsulates r’s reference. The PhantomReference object behaves as a phantom reference to r. The ReferenceQueue object identified by q is associated with this PhantomReference object. Passing null to q makes no sense because get() is overridden to return null and the PhantomReference object will never be enqueued. Unlike WeakReference and SoftReference objects, which are enqueued onto their reference queues when their referents become weakly reachable (before finalization), or sometime after their referents become softly reachable (before finalization), PhantomReference objects are enqueued after their referents have been reclaimed. Although you cannot access a PhantomReference object’s referent (its get() method returns null), this class is useful because enqueuing the PhantomReference object tells you exactly when the referent has been removed. Perhaps you want to delay creating a large object until another large object has been removed (to avoid a thrown java.lang.OutOfMemoryError object). PhantomReference is also useful as a substitute for resurrection (making an unreachable object reachable). Because there is no way to access the referent (get() returns null), which is no longer in memory when the PhantomReference object is enqueued, the object can be cleaned up during the first garbage collection cycle in which that object was discovered to be phantom reachable. You can then clean up related resources after receiving notification via the PhantomReference object’s reference queue. NOTE: Resurrection occurs in the finalize() method when you assign this to a root-set variable. For example, you might specify r = this; within finalize() to assign the unreachable object identified as this to a class field named r. In contrast, the garbage collector requires at least two garbage collection cycles to determine if an object that overrides finalize() can be garbage collected. When the first cycle detects that the object is eligible for garbage collection, it calls finalize(). Because this method might have resurrected the object, a second garbage collection cycle is needed to determine if resurrection has happened. CAUTION: Resurrection has been used to implement object pools that recycle the same objects when these objects are expensive (time-wise) to create (database connection objects are an example). Because resurrection exacts a severe performance penalty, and because the PhantomReference class makes resurrection unnecessary, you should avoid using resurrection in your applications. Listing 6–15 shows how you might use PhantomReference to detect the removal of a large object. This book was purchased by harshamv@variable3.comCHAPTER 6: Exploring the Basic APIs Part 1 265 Listing 6–15. Detecting a large object’s removal class LargeObject { private byte[] memory = new byte[1024*1024*50]; // 50 megabytes } public class LargeObjectDemo { public static void main(String[] args) { ReferenceQueue rq; rq = new ReferenceQueue(); PhantomReference pr; pr = new PhantomReference(new LargeObject(), rq); int counter = 0; int[] x; while (rq.poll() == null) { System.out.println("waiting for large object to be removed"); if (counter++ == 10) x = new int[1024*1024]; } System.out.println("large object removed"); } } Listing 6–15 declares a LargeObject class whose private memory array occupies 50MB. If your Java implementation throws OutOfMemoryError when you run this application, you might need to reduce the size of this array. The main() method first creates a ReferenceQueue object that describes a queue onto which a subsequently created PhantomReference object that contains a LargeObject reference will be enqueued. main() next creates the PhantomReference object, passing a reference to a newly created LargeObject object and a reference to the previously created ReferenceQueue object to the constructor. After initializing a counter variable (which determines how many loop iterations pass before another large object is created), and after introducing a local variable named x that will hold a strong reference to another large object, main() enters a polling loop. The polling loop begins by calling poll() to detect the removal of the LargeObject object from memory. As long as this method returns null, meaning that the LargeObject object is still in memory, the loop outputs a message and increments counter. When counter’s value reaches 10, x is assigned an int-based array containing one million integer elements. Because the reference stored in x is strong, this array will not be garbage collected (before the application ends). On my platform, assigning this array’s reference to x is sufficient for the garbage collector to destroy the LargeObject object. Its PhantomReference object is enqueued onto the rq-referenced ReferenceQueue; poll() returns the PhantomReference object. CHAPTER 6: Exploring the Basic APIs Part 1 266 Depending on your implementation of the virtual machine, you might or might not observe the large object removed message. If you do not see this message, you might need to increase the size of array x, making sure that OutOfMemoryError is not thrown. When I run this application on my platform, I observe the following output—you may have to adjust the application’s code to observe similar output: waiting for large object to be removed waiting for large object to be removed waiting for large object to be removed waiting for large object to be removed waiting for large object to be removed waiting for large object to be removed waiting for large object to be removed waiting for large object to be removed waiting for large object to be removed waiting for large object to be removed waiting for large object to be removed large object removed NOTE: For a more useful example of PhantomReference, and for more in-depth knowledge of garbage collection, check out Keith D Gregory’s “Java Reference Objects” blog post (http://www.kdgregory.com/index.php?page=java.refobj). EXERCISES The following exercises are designed to test your understanding of Java’s basic APIs: 1. What constants does Math declare? 2. Why is Math.abs(Integer.MIN_VALUE) equal to Integer.MIN_VALUE? 3. What does Math’s random() method accomplish? 4. Identify the five special values that can arise during floating-point calculations. 5. How do Math and StrictMath differ? 6. What is the purpose of strictfp? 7. What is BigDecimal and why might you use this class? 8. Which RoundingMode constant describes the form of rounding commonly taught at school? 9. What is BigInteger? 10. What is the purpose of Package’s isSealed() method? 11. True or false: getPackage() requires at least one classfile to be loaded from the package before it returns a Package object describing that package. 12. Identify the two main uses of the primitive wrapper classes. CHAPTER 6: Exploring the Basic APIs Part 1 267 13. Why should you avoid coding expressions such as ch >= '0' && ch <= '9' (test ch to see if it contains a digit) or ch >= 'A' && ch <= 'Z' (test ch to see if it contains an uppercase letter)? 14. Identify the four kinds of reachability. 15. What is a referent? 16. Which of the References API’s classes is the equivalent of Object’s finalize() method? 17. Before the era of graphics screens, developers sometimes used a text-based screen to display graphics shapes. For example, a circle might be displayed as follows: * ********* ** ** ** ** * * * * ** ** * * * * * * ** ** * * * * * * ** ** * * * * ** ** ** ** ********* * NOTE: This shape appears elliptical instead of circular because each asterisk’s displayed height is greater than its displayed width. If the height and width matched, the shape would appear circular. Create a Circle application that generates and displays the previous circle shape. Start by creating a two-dimensional screen array of 22 rows by 22 columns. Initialize each array element to the space character (indicating a clear screen). For each integer angle from 0 to 360, compute the x and y coordinates by multiplying a radius value of 10 by each of the cosine and sine of the angle. Add 11 to the x value and 11 to the y value to center the circle shape within the screen array. Assign an asterisk to the array at the resulting (x, y) coordinates. After the loop completes, output the array to the standard output device. 18. A prime number is a positive integer greater than 1 that is evenly divisible only by 1 and itself. Create a PrimeNumberTest application that determines if its solitary integer argument is prime or not prime, and outputs a suitable message. For example, java PrimeNumberTest 289 should output the message 289 is not prime. A simple way to check for primality is to loop from 2 through the square root of the integer argument, and use the remainder operator in the loop to determine if the argument is divided evenly by the loop index. For example, because 6%2 yields a remainder of 0 (2 divides evenly into 6), integer 6 is not a prime number. CHAPTER 6: Exploring the Basic APIs Part 1 268 Summary The java.lang.Math class supplements the basic math operations (+, -, *, /, and %) with advanced operations (such as trigonometry). The companion java.lang.StrictMath class ensures that all of these operations yield the same values on all platforms. Money must never be represented by floating-point and double precision floating-point variables because not all monetary values can be represented exactly. In contrast, the java.math.BigDecimal class lets you accurately represent and manipulate these values. BigDecimal relies on the java.math.BigInteger class for representing its unscaled value. A BigInteger instance describes an integer value that can be of arbitrary length (subject to the limits of the virtual machine’s memory). The java.lang.Package class provides access to package information. This information includes version information about the implementation and specification of a Java package, the package’s name, and an indication of whether the package is sealed or not. Instances of the java.lang package’s Boolean, Byte, Character, Double, Float, Integer, Long, and Short primitive wrapper classes wrap themselves around values of primitive types. These classes are useful for storing primitive values in collections. The References API makes it possible for an application to interact with the garbage collector in limited ways. This API’s java.lang.ref package contains classes Reference, ReferenceQueue, SoftReference, WeakReference, and PhantomReference. SoftReference is useful for implementing image caches, WeakReference is useful for preventing memory leaks related to hashmaps, and PhantomReference is useful for learning when an object has died so its resources can be cleaned up. Your exploration of Java’s basic APIs is far from finished. Chapter 7 continues to focus on basic APIs by discussing the Reflection API, string management, the System class, and the low-level Threading API. 269 269 Chapter Exploring the Basic APIs Part 2 Chapter 7 continues to explore Java’s basic (language-oriented) APIs by introducing APIs that let you use reflection to obtain type information at runtime and more, manage strings, perform system activities (such as retrieving a system property value and obtaining the current time), and use threads to improve application performance. NOTE: Chapter 7 explores basic API classes and interfaces that are located in the java.lang and java.lang.reflect packages. Reflection API Chapter 3 referred to reflection as a third form of runtime type identification (RTTI). Java’s Reflection API lets applications learn about loaded classes, interfaces, enums (a kind of class), and annotation types (a kind of interface). The API also lets applications instantiate classes, call methods, access fields, and perform other tasks reflectively. Chapter 5 presented a StubFinder application that used part of the Reflection API to load a class and identify all of the loaded class’s public methods that are annotated with @Stub annotations. This tool is one example where using reflection is beneficial. Another example is the class browser, a tool that enumerates the members of a class. CAUTION: Reflection should not be used indiscriminately. Application performance suffers because it takes longer to perform operations with reflection than without reflection. Also, reflection-oriented code can be harder to read, and the absence of compile-time type checking can result in runtime failures. 7 CHAPTER 7: Exploring the Basic APIs Part 2 270 The java.lang package’s Class class is the entry point into the Reflection API. Class is generically declared as Class, where T identifies the class, interface, enum, or annotation type that is being modeled by the Class object. T can be replaced by ? (as in Class) when the type being modeled is unknown. Table 7–1 describes some of Class’s methods. Table 7–1. Class Methods Method Description static Class forName(String typename) Return the Class object that is associated with typename, which must include the type’s qualified package name when the type is part of a package (java.lang.String, for example). If the class or interface type has not been loaded into memory, this method takes care of loading (reading the classfile’s contents into memory), linking (taking these contents and combining them into the runtime state of the virtual machine so that they can be executed), and initializing (setting class fields to default values, running class initializers, and performing other class initialization) prior to returning the Class object. This method throws java.lang.ClassNotFoundException when the type cannot be found, java.lang.LinkageError when an error occurs during linkage, and java.lang.ExceptionInInitializerError when an exception occurs during a class’s static initialization. Annotation[] getAnnotations() Return a possibly empty array containing all annotations that are declared for the class represented by this Class object. Constructor[] getConstructors() Return an array containing Constructor objects representing all the public constructors of the class represented by this Class object. An array of length zero is returned when the represented class has no public constructors, this Class object represents an array class, or this Class object represents a primitive type or void. Annotation[] getDeclaredAnnotations() Return an array containing all annotations that are directly declared on the class represented by this Class object— inherited annotations are not included. The returned array might be empty. Constructor[] getDeclaredConstructors() Return an array of Constructor objects representing all the constructors declared by the class represented by this Class object. These are public, protected, default (package) access, and private constructors. The elements in the returned array are not sorted and are not in any particular order. If the class has a default constructor, it is included in the returned array. This method returns an array of length zero when this Class object represents an interface, a primitive type, an array class, or void. CHAPTER 7: Exploring the Basic APIs Part 2 271 Method Description Field[] getDeclaredFields() Return an array of Field objects representing all the fields declared by the class or interface represented by this Class object. This array includes public, protected, default (package) access, and private fields, but excludes inherited fields. The elements in the returned array are not sorted and are not in any particular order. This method returns an array of length zero when the class or interface declares no fields, or when this Class object represents a primitive type, an array class, or void. Method[] getDeclaredMethods() Return an array of Method objects representing all the methods declared by the class or interface represented by this Class object. This array includes public, protected, default (package) access, and private methods, but excludes inherited methods. The elements in the returned array are not sorted and are not in any particular order. This method returns an array of length zero when the class or interface declares no methods, or when this Class object represents a primitive type, an array class, or void. Field[] getFields() Return an array containing Field objects representing all the public fields of the class or interface represented by this Class object, including those public fields inherited from superclasses and superinterfaces. The elements in the returned array are not sorted and are not in any particular order. This method returns an array of length zero when this Class object represents a class or interface that has no accessible public fields, or when this Class object represents an array class, a primitive type, or void. Method[] getMethods() Return an array containing Method objects representing all the public methods of the class or interface represented by this Class object, including those public methods inherited from superclasses and superinterfaces. Array classes return all the public member methods inherited from the Object class. The elements in the returned array are not sorted and are not in any particular order. This method returns an array of length zero when this Class object represents a class or interface that has no public methods, or when this Class object represents a primitive type or void. String getName() Return the name of the class represented by this Class object. Package getPackage() Return a Package object that describes the package in which the class represented by this Class object is located, or null when the class is a member of the unnamed package. CHAPTER 7: Exploring the Basic APIs Part 2 272 Method Description Class getSuperclass() Return the Class object representing the superclass of the entity (class, interface, primitive type, or void) represented by this Class object. When the Class object on which this method is called represents the Object class, an interface, a primitive type, or void, null is returned. When this object represents an array class, the Class object representing the Object class is returned. T newInstance() Create and return a new instance of the class represented by this Class object. The class is instantiated as if by a new expression with an empty argument list. The class is initialized when it has not already been initialized. This method throws java.lang.IllegalAccessException when the class or its noargument constructor is not accessible; java.lang.InstantiationException when this Class object represents an abstract class, an interface, an array class, a primitive type, or void, or when the class does not have a noargument constructor (or when instantiation fails for some other reason); and ExceptionInInitializerError when initialization fails because the object threw an exception during initialization. Table 7–1’s description of the forName() method reveals one way to obtain a Class object. This method loads, links, and initializes a class or interface that is not in memory and returns a Class object that represents the class or interface. Listing 7–1 demonstrates forName() and additional methods described in this table. Listing 7–1. Using reflection to identify a class’s name, package, public fields, constructors, and methods import java.lang.reflect.Constructor; import java.lang.reflect.Field; import java.lang.reflect.Method; public class ExploreType { public static void main(String[] args) { if (args.length != 1) { System.err.println("usage: java ExploreType pkgAndTypeName"); return; } try { Class clazz = Class.forName(args[0]); System.out.println("NAME: " + clazz.getName()); System.out.println("PACKAGE: " + clazz.getPackage().getName()); System.out.println("FIELDS"); Field[] fields = clazz.getDeclaredFields(); for (int i = 0; i < fields.length; i++) System.out.println(fields[i]); CHAPTER 7: Exploring the Basic APIs Part 2 273 System.out.println("CONSTRUCTORS"); Constructor[] constructors = clazz.getDeclaredConstructors(); for (int i = 0; i < constructors.length; i++) System.out.println(constructors[i]); System.out.println("METHODS"); Method[] methods = clazz.getDeclaredMethods(); for (int i = 0; i < methods.length; i++) System.out.println(methods[i]); } catch (ClassNotFoundException cnfe) { System.err.println("could not locate " + args[0]); } } } Listing 7–1 presents an application that uses the Reflection API to explore a class or interface by outputting its name, package, fields, constructors (classes only), and methods. Only fields, constructors, and methods that are declared in the class, or fields and methods that are declared in the interface, are output. After verifying that one command-line argument has been passed to this application, main() calls forName() to try to return a Class object representing the class or interface identified by this argument. If successful, the returned object’s reference is assigned to clazz—I cannot name this variable class because class is a reserved word. forName() throws an instance of the checked ClassNotFoundException class when it cannot locate the class’s classfile (perhaps the classfile was erased prior to executing the application). It also throws LinkageError when a class’s classfile is malformed, and ExceptionInInitializerError when a class’s static initialization fails. NOTE: ExceptionInInitializerError is often thrown as the result of a class initializer throwing an unchecked exception. For example, the class initializer in the following FailedInitialization class results in ExceptionInInitializerError because someMethod() throws NullPointerException: public class FailedInitialization { static { someMethod(null); } public static void someMethod(String s) { int len = s.length(); // s contains null System.out.println(s + "'s length is " + len + " characters"); } } CHAPTER 7: Exploring the Basic APIs Part 2 274 When you run this application, you must include the package specification when the class or interface is located in a package. For example, specifying java ExploreType java.lang.Boolean to output the fields, constructors, and methods declared in the java.lang package’s Boolean class results in the following output: NAME: java.lang.Boolean PACKAGE: java.lang FIELDS public static final java.lang.Boolean java.lang.Boolean.TRUE public static final java.lang.Boolean java.lang.Boolean.FALSE public static final java.lang.Class java.lang.Boolean.TYPE private final boolean java.lang.Boolean.value private static final long java.lang.Boolean.serialVersionUID CONSTRUCTORS public java.lang.Boolean(java.lang.String) public java.lang.Boolean(boolean) METHODS public int java.lang.Boolean.hashCode() public boolean java.lang.Boolean.equals(java.lang.Object) public int java.lang.Boolean.compareTo(java.lang.Boolean) public int java.lang.Boolean.compareTo(java.lang.Object) public static boolean java.lang.Boolean.getBoolean(java.lang.String) public static java.lang.String java.lang.Boolean.toString(boolean) public java.lang.String java.lang.Boolean.toString() public static java.lang.Boolean java.lang.Boolean.valueOf(java.lang.String) public static java.lang.Boolean java.lang.Boolean.valueOf(boolean) public boolean java.lang.Boolean.booleanValue() public static boolean java.lang.Boolean.parseBoolean(java.lang.String) private static boolean java.lang.Boolean.toBoolean(java.lang.String) Table 7–1’s descriptions of the getAnnotations() and getDeclaredAnnotations() methods reveal that each method returns an array of Annotation, an interface that is located in the java.lang.annotation package. Annotation is the superinterface of Override, SuppressWarnings, and all other annotation types. Table 7–1’s method descriptions also refer to Constructor, Field, and Method. Instances of these classes (which are members of the java.lang.reflect package) represent a class’s constructors and a class’s or an interface’s fields and methods. Constructor represents a constructor and is generically declared as Constructor, where T identifies the class in which the constructor represented by Constructor is declared. Constructor declares various methods, including the following methods:  Annotation[] getDeclaredAnnotations() returns an array of all annotations declared on the constructor. The returned array has zero length when there are no annotations.  Class getDeclaringClass() returns a Class object that represents the class in which the constructor is declared. This book was purchased by harshamv@variable3.comCHAPTER 7: Exploring the Basic APIs Part 2 275  Class[] getExceptionTypes() returns an array of Class objects representing the types of exceptions listed in the constructor’s throws clause. The returned array has zero length when there is no throws clause.  String getName() returns the constructor’s name.  Class[] getParameterTypes() returns an array of Class objects representing the constructor’s parameters. The returned array has zero length when the constructor does not declare parameters. Field represents a field and declares various methods, including the following methods:  Object get(Object object) returns the value of the field for the specified object.  boolean getBoolean(Object object) returns the value of the Boolean field for the specified object.  byte getByte(Object object) returns the value of the byte integer field for the specified object.  char getChar(Object object) returns the value of the character field for the specified object.  double getDouble(Object object) returns the value of the double precision floating-point field for the specified object.  float getFloat(Object object) returns the value of the floating-point field for the specified object.  int getInt(Object object) returns the value of the integer field for the specified object.  long getLong(Object object) returns the value of the long integer field for the specified object.  short getShort(Object object) returns the value of the short integer field for the specified object. get() returns the value of any type of field. In contrast, the other listed methods return the values of specific types of fields. All of these methods throw NullPointerException when object is null and the field is an instance field, IllegalArgumentException when object is not an instance of the class or interface declaring the underlying field (or not an instance of a subclass or interface implementor), and IllegalAccessException when the underlying field cannot be accessed (it is private, for example). Method represents a method and declares various methods, including the following methods: CHAPTER 7: Exploring the Basic APIs Part 2 276  int getModifiers() returns a 32-bit integer whose bit fields identify the method’s reserved word modifiers (such as public, abstract, or static). These bit fields must be interpreted via the java.lang.reflect.Modifier class. For example, you might specify (method.getModifiers() & Modifier.ABSTRACT) == Method.ABSTRACT to find out if the method (represented by the Method object whose reference is stored in method) is abstract—this expression evaluates to true when the method is abstract.  Class getReturnType() returns a Class object that represents the method’s return type.  Object invoke(Object receiver, Object... args) calls the method on the object identified by receiver (which is ignored when the method is a class method), passing the variable number of arguments identified by args to the called method. The invoke() method throws NullPointerException when receiver is null and the method being called is an instance method, IllegalAccessException when the method is not accessible (it is private, for example), IllegalArgumentException when an incorrect number of arguments are passed to the method (and other reasons), and java.lang.reflect.InvocationTargetException when an exception is thrown from the called method.  boolean isVarArgs() returns true when the method is declared to receive a variable number of arguments. The java.lang.reflect.AccessibleObject class is the superclass of Constructor, Field, and Method. This superclass provides methods for reporting a constructor’s, field’s, or method’s accessibility (is it private?) and making an inaccessible constructor, field, or method accessible. AccessibleObject’s methods include the following:  T getAnnotation(Class annotationType) returns the constructor’s, field’s, or method’s annotation of the specified type when such an annotation is present; otherwise, null returns.  boolean isAccessible() returns true when the constructor, field, or method is accessible.  boolean isAnnotationPresent(Class annotationType) returns true when an annotation of the type specified by annotationType has been declared on the constructor, field, or method. This method takes inherited annotations into account.  void setAccessible(boolean flag) attempts to make an inaccessible constructor, field, or method accessible when flag is true. NOTE: The java.lang.reflect package also includes an Array class whose class methods make it possible to reflectively create and access Java arrays. CHAPTER 7: Exploring the Basic APIs Part 2 277 Another way to obtain a Class object is to call Object’s getClass() method on an object reference; for example, Employee e = new Employee(); Class clazz = e.getClass();. The getClass() method does not throw an exception because the class from which the object was created is already present in memory. There is one more way to obtain a Class object, and that is to employ a class literal, which is an expression consisting of a class name, followed by a period separator, followed by reserved word class. Examples of class literals include Class clazz = Employee.class; and Class clazz = String.class. Perhaps you are wondering about how to choose between forName(), getClass(), and a class literal. To help you make your choice, the following list compares each competitor:  forName() is very flexible in that you can dynamically specify any reference type by its package-qualified name. If the type is not in memory, it is loaded, linked, and initialized. However, lack of compile- time type safety can lead to runtime failures.  getClass() returns a Class object describing the type of its referenced object. If called on a superclass variable containing a subclass instance, a Class object representing the subclass type is returned. Because the class is in memory, type safety is assured.  A class literal returns a Class object representing its specified class. Class literals are compact and the compiler enforces type safety by refusing to compile the source code when it cannot locate the literal’s specified class. NOTE: You can use class literals with primitive types, including void. Examples include int.class, double.class, and void.class. The returned Class object represents the class identified by a primitive wrapper class’s TYPE field or java.lang.Void.TYPE. For example, each of int.class == Integer.TYPE and void.class == Void.TYPE evaluates to true. You can also use class literals with primitive type–based arrays. Examples include int[].class and double[].class. For these examples, the returned Class objects represent Class and Class. String Management Many computer languages implement the concept of a string, a sequence of characters treated as a single unit (and not as individual characters). For example, the C language implements a string as an array of characters terminated by the null character ('\0'). In contrast, Java implements a string via the java.lang.String class. CHAPTER 7: Exploring the Basic APIs Part 2 278 String objects are immutable: you cannot modify a String object’s string. The various String methods that appear to modify the String object actually return a new String object with modified string content instead. Because returning new String objects is often wasteful, Java provides the java.lang.StringBuffer class as a workaround. This section introduces you to String and StringBuffer. String String represents a string as a sequence of characters. Unlike C strings, this sequence is not terminated by a null character. Instead, its length is stored separately. The Java language provides syntactic sugar that simplifies working with strings. For example, the compiler recognizes String favLanguage = "Java"; as the assignment of string literal "Java" to String variable favLanguage. Without this sugar, you would have to specify String favLanguage = new String("Java");. Table 7–2 describes some of String’s constructors and methods for initializing String objects and working with strings. Table 7–2. String Constructors and Methods Method Description String(char[] data) Initialize this String object to the characters in the data array. Modifying data after initializing this String object has no effect on the object. String(String s) Initialize this String object to s’s string. char charAt(int index) Return the character located at the zero-based index in this String object’s string. This method throws java.lang.IndexOutOfBoundsException when index is less than 0 or greater than or equal to the length of the string. String concat(String s) Return a new String object containing this String object’s string followed by the s argument’s string. boolean endsWith(String suffix) Return true when this String object’s string ends with the characters in the suffix argument, when suffix is empty (contains no characters), or when suffix contains the same character sequence as this String object’s string. This method performs a case-sensitive comparison (a is not equal to A, for example), and throws NullPointerException when suffix is null. boolean equals(Object object) Return true when object is of type String and this argument’s string contains the same characters (and in the same order) as this String object’s string. CHAPTER 7: Exploring the Basic APIs Part 2 279 Method Description boolean equalsIgnoreCase(String s) Return true when s and this String object contain the same characters (ignoring case). This method returns false when the character sequences differ or when null is passed to s. int indexOf(int c) Return the zero-based index of the first occurrence (from the start of the string to the end of the string) of the character represented by c in this String object’s string. Return -1 when this character is not present. int indexOf(String s) Return the zero-based index of the first occurrence (from the start of the string to the end of the string) of s’s character sequence in this String object’s string. Return -1 when s is not present. This method throws NullPointerException when s is null. String intern() Search an internal table of String objects for an object whose string is equal to this String object’s string. This String object’s string is added to the table when not present. Return the object contained in the table whose string is equal to this String object’s string. The same String object is always returned for strings that are equal. int lastIndexOf(int c) Return the zero-based index of the last occurrence (from the start of the string to the end of the string) of the character represented by c in this String object’s string. Return -1 when this character is not present. int lastIndexOf(String s) Return the zero-based index of the last occurrence (from the start of the string to the end of the string) of s’s character sequence in this String object’s string. Return -1 when s is not present. This method throws NullPointerException when s is null. int length() Return the number of characters in this String object’s string. String replace(char oldChar, char newChar) Return a new String object whose string matches this String object’s string except that all occurrences of oldChar have been replaced by newChar. String[] split(String expr) Split this String object’s string into an array of String objects using the regular expression (a string whose pattern [template] is used to search a string for substrings that match the pattern) specified by expr as the basis for the split. This method throws NullPointerException when expr is null and java.util.regex.PatternSyntaxException when expr’s syntax is invalid. CHAPTER 7: Exploring the Basic APIs Part 2 280 Method Description boolean startsWith(String prefix) Return true when this String object’s string starts with the characters in the prefix argument, when prefix is empty (contains no characters), or when prefix contains the same character sequence as this String object’s string. This method performs a case-sensitive comparison (a is not equal to A, for example), and throws NullPointerException when prefix is null. String substring(int start) Return a new String object whose string contains this String object’s characters beginning with the character located at start. This method throws IndexOutOfBoundsException when start is negative or greater than the length of this String object’s string. char[] toCharArray() Return a character array that contains the characters in this String object’s string. String toLowerCase() Return a new String object whose string contains this String object’s characters where uppercase letters have been converted to lowercase. This String object is returned when it contains no uppercase letters to convert. String toUpperCase() Return a new String object whose string contains this String object’s characters where lowercase letters have been converted to uppercase. This String object is returned when it contains no lowercase letters to convert. String trim() Return a new String object that contains this String object’s string with whitespace characters (characters whose Unicode values are 32 or less) removed from the start and end of the string, or this String object if no leading/trailing whitespace. Table 7–2 reveals a couple of interesting items about String. First, this class’s public String(String s) constructor does not initialize a String object to a string literal. Instead, it initializes the String object to the contents of another String object. This behavior suggests that a string literal is more than what it appears to be. In reality, a string literal is a String object. You can prove this to yourself by executing System.out.println("abc".length()); and System.out.println("abc" instanceof String);. The first method call outputs 3, which is the length of the "abc" String object’s string, and the second method call outputs true ("abc" is a String object). The second interesting item is the intern() method, which interns (stores a unique copy of) a String object in an internal table of String objects. intern() makes it possible to compare strings via their references and == or !=. These operators are the fastest way to compare strings, which is especially valuable when sorting a huge number of strings. CHAPTER 7: Exploring the Basic APIs Part 2 281 By default, String objects denoted by literal strings ("abc") and string-valued constant expressions ("a" + "bc") are interned in this table, which is why System.out.println("abc" == "a" + "bc"); outputs true. However, String objects created via String constructors are not interned, which is why System.out.println("abc" == new String("abc")); outputs false. In contrast, System.out.println("abc" == new String("abc").intern()); outputs true. Table 7–2 also reveals split(), a method that I employed in Chapter 5’s StubFinder application to split a string’s comma-separated list of values into an array of String objects. This method uses a regular expression that identifies a sequence of characters around which the string is split. (I will discuss regular expressions in Chapter 9.) TIP: The charAt() and length() methods are useful for iterating over a string’s characters. For example, String s = "abc"; for (int i = 0; i < s.length(); i++) System.out.println(s.charAt(i)); returns each of s’s a, b, and c characters and outputs each character on a separate line. StringBuffer StringBuffer provides an internal character array for building a string efficiently. After creating a StringBuffer object, you call various methods to append, delete, and insert the character representations of various values to, from, and into the array. You then call toString() to convert the array’s content to a String object and return this object. CAUTION: Divulging a class’s internal implementation typically is not a good idea because doing so violates information hiding. Furthermore, the internal implementation might change, which voids the description of the previous implementation. However, I believe that divulging StringBuffer’s internal array adds value to my discussion of this class. Furthermore, it is highly unlikely that StringBuffer will ever use anything other than a character array. Table 7–3 describes some of StringBuffer’s constructors and methods for initializing StringBuffer objects and working with string buffers. CHAPTER 7: Exploring the Basic APIs Part 2 282 Table 7–3. StringBuffer Constructors and Methods Method Description StringBuffer() Initialize this StringBuffer object to an empty array with an initial capacity of 16 characters. StringBuffer(int capacity) Initialize this StringBuffer object to an empty array with an initial capacity of capacity characters. This constructor throws java.lang.NegativeArraySizeException when capacity is negative. StringBuffer(String s) Initialize this StringBuffer object to an array containing s’s characters. This object’s initial capacity is 16 plus the length of s. This constructor throws NullPointerException when s is null. StringBuffer append(boolean b) Append “true” to this StringBuffer object’s array when b is true and “false” to the array when b is false, and return this StringBuffer object. StringBuffer append(char ch) Append ch’s character to this StringBuffer object’s array, and return this StringBuffer object. StringBuffer append(char[] chars) Append the characters in the chars array to this StringBuffer object’s array, and return this StringBuffer object. This method throws NullPointerException when chars is null. StringBuffer append(double d) Append the string representation of d’s double precision floating- point value to this StringBuffer object’s array, and return this StringBuffer object. StringBuffer append(float f) Append the string representation of f’s floating-point value to this StringBuffer object’s array, and return this StringBuffer object. StringBuffer append(int i) Append the string representation of i’s integer value to this StringBuffer object’s array, and return this StringBuffer object. StringBuffer append(long l) Append the string representation of l’s long integer value to this StringBuffer object’s array, and return this StringBuffer object. StringBuffer append(Object obj) Call obj’s toString() method and append the returned string’s characters to this StringBuffer object’s array. Append “null” to the array when null is passed to obj. Return this StringBuffer object. StringBuffer append(String s) Append s’s string to this StringBuffer object’s array. Append “null” to the array when null is passed to s. Return this StringBuffer object. CHAPTER 7: Exploring the Basic APIs Part 2 283 Method Description int capacity() Return the current capacity of this StringBuffer object’s array. char charAt(int index) Return the character located at index in this StringBuffer object’s array. This method throws IndexOutOfBoundsException when index is negative or greater than or equal to this StringBuffer object’s length. void ensureCapacity(int min) Ensure that this StringBuffer object’s capacity is at least that specified by min. If the current capacity is less than min, a new internal array is created with greater capacity. The new capacity is set to the larger of min and the current capacity multiplied by 2, with 2 added to the result. No action is taken when min is negative or zero. int length() Return the number of characters stored in this StringBuffer object’s array. StringBuffer reverse() Return this StringBuffer object with its array contents reversed. void setCharAt(int index, char ch) Replace the character at index with ch. This method throws IndexOutOfBoundsException when index is negative or greater than or equal to the length of this StringBuffer object’s array. void setLength(int length) Set the length of this StringBuffer object’s array to length. If the length argument is less than the current length, the array’s contents are truncated. If the length argument is greater than or equal to the current length, sufficient null characters ('\u0000') are appended to the array. This method throws IndexOutOfBoundsException when length is negative. String substring(int start) Return a new String object that contains all characters in this StringBuffer object’s array starting with the character located at start. This method throws IndexOutOfBoundsException when start is less than 0 or greater than or equal to the length of this StringBuffer object’s array. String toString() Return a new String object whose string equals the contents of this StringBuffer object’s array. A StringBuffer object’s internal array is associated with the concepts of capacity and length. Capacity refers to the maximum number of characters that can be stored in the array before the array grows to accommodate additional characters. Length refers to the number of characters that are already stored in the array. Chapter 6’s Listing 6-13 declared a toAlignedBinaryString() method whose implementation included the following inefficient loop: int numLeadingZeroes = 3; CHAPTER 7: Exploring the Basic APIs Part 2 284 String zeroesPrefix = ""; for (int j = 0; j < numLeadingZeroes; j++) zeroesPrefix += "0"; This loop is inefficient because each of the iterations creates a StringBuffer object and a String object. The compiler transforms this code fragment into the following fragment: int numLeadingZeroes = 3; String zeroesPrefix = ""; for (int j = 0; j < numLeadingZeroes; j++) zeroesPrefix = new StringBuffer().append(zeroesPrefix).append("0").toString(); NOTE: Starting with Java version 5, the compiler uses the more performant but otherwise identical java.lang.StringBuilder class instead of StringBuffer. A more efficient way to code the previous loop involves creating a StringBuffer object prior to entering the loop, calling the appropriate append() method in the loop, and calling toString() after the loop. The following code fragment demonstrates this more efficient scenario: int numLeadingZeroes = 3; StringBuffer sb = new StringBuffer(); for (int j = 0; j < numLeadingZeroes; j++) sb.append('0'); String zeroesPrefix = sb.toString(); TIP: When performance matters, and where you are not using multiple threads (discussed later in this chapter), use the StringBuilder class instead of StringBuffer. StringBuilder provides the same methods as StringBuffer, but its methods are not synchronized (discussed later in this chapter). This lack of synchronization results in StringBuilder methods executing faster than their StringBuffer counterparts under most (if not all) Java implementations. System The java.lang.System class provides access to system-oriented resources, including standard input, standard output, and standard error. System declares in, out, and err class fields that support standard input, standard output, and standard error, respectively. The first field is of type java.io.InputStream, and the last two fields are of type java.io.PrintStream. (I will formally introduce these classes in Chapter 10.) System also declares a variety of utility methods, including those methods that are described in Table 7–4. This book was purchased by harshamv@variable3.comCHAPTER 7: Exploring the Basic APIs Part 2 285 Table 7–4. System Methods Method Description static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length) Copy the number of elements specified by length from the src array starting at zero-based offset srcPos into the dest array starting at zero-based offset destPos. This method throws NullPointerException when src or dest is null, IndexOutOfBoundsException when copying causes access to data outside array bounds, and java.lang.ArrayStoreException when an element in the src array could not be stored into the dest array because of a type mismatch. static long currentTimeMillis() Return the current system time in milliseconds since January 1, 1970 00:00:00 UTC. static void gc() Inform the virtual machine that now would be a good time to run the garbage collector. This is only a hint; there is no guarantee that the garbage collector will run. static String getProperty(String prop) Return the value of the system property (platform-specific attribute, such as a version number) identified by prop or null if such a property does not exist. System properties recognized by Android include java.vendor.url, java.class.path, user.home, java.class.version, os.version, java.vendor, user.dir, user.timezone, path.separator, os.name, os.arch, line.separator, file.separator, user.name, java.version, and java.home. static void runFinalization() Inform the virtual machine that now would be a good time to perform any outstanding object finalizations. This is only a hint; there is no guarantee that outstanding object finalizations will be performed. Listing 7–2 demonstrates the arraycopy(), currentTimeMillis(), and getProperty() methods. Listing 7–2. Experimenting with System methods public class SystemTasks { public static void main(String[] args) { int[] grades = { 86, 92, 78, 65, 52, 43, 72, 98, 81 }; int[] gradesBackup = new int[grades.length]; System.arraycopy(grades, 0, gradesBackup, 0, grades.length); for (int i = 0; i < gradesBackup.length; i++) System.out.println(gradesBackup[i]); System.out.println("Current time: " + System.currentTimeMillis()); String[] propNames = { "java.vendor.url", "java.class.path", "user.home", CHAPTER 7: Exploring the Basic APIs Part 2 286 "java.class.version", "os.version", "java.vendor", "user.dir", "user.timezone", "path.separator", "os.name", "os.arch", "line.separator", "file.separator", "user.name", "java.version", "java.home" }; for (int i = 0; i < propNames.length; i++) System.out.println(propNames[i] + ": " + System.getProperty(propNames[i])); } } Listing 7–2’s main() method begins by demonstrating arraycopy(). It uses this method to copy the contents of a grades array to a gradesBackup array. TIP: The arraycopy() method is the fastest portable way to copy one array to another. Also, when you write a class whose methods return a reference to an internal array, you should use arraycopy() to create a copy of the array, and then return the copy’s reference. That way, you prevent clients from directly manipulating (and possibly screwing up) the internal array. main() next calls currentTimeMillis() to return the current time as a milliseconds value. Because this value is not human-readable, you might want to use the java.util.Date class (discussed in Chapter 9). The Date() constructor calls currentTimeMillis() and its toString() method converts this value to a readable date and time. main() concludes by demonstrating getProperty() in a for loop. This loop iterates over all of Table 7–4’s property names, outputting each name and value. When I run this application on my platform, it generates the following output: 86 92 78 65 52 43 72 98 81 Current time: 1274895119343 java.vendor.url: http://java.sun.com/ java.class.path: . user.home: C:\Documents and Settings\Jeff Friesen java.class.version: 50.0 os.version: 5.1 CHAPTER 7: Exploring the Basic APIs Part 2 287 java.vendor: Sun Microsystems Inc. user.dir: C:\prj\dev\ljfad\c06\code\SYSTEM~1 user.timezone: path.separator: ; os.name: Windows XP os.arch: x86 line.separator: file.separator: \ user.name: Jeff Friesen java.version: 1.6.0_16 java.home: C:\Program Files\Java\jre6 NOTE: line.separator stores the actual line separator character/characters, not its/their representation (such as \r\n), which is why a blank line appears after line.separator:. Threading API Applications execute via threads, which are independent paths of execution through an application’s code. When multiple threads are executing, each thread’s path can differ from other thread paths. For example, a thread might execute one of a switch statement’s cases, and another thread might execute another of this statement’s cases. NOTE: Applications use threads to improve performance. Some applications can get by with only the default main thread to carry out their tasks, but other applications need additional threads to perform time-intensive tasks in the background, so that they remain responsive to their users. The virtual machine gives each thread its own method-call stack to prevent threads from interfering with each other. Separate stacks let threads keep track of their next instructions to execute, which can differ from thread to thread. The stack also provides a thread with its own copy of method parameters, local variables, and return value. Java supports threads via its Threading API. This API consists of one interface (Runnable) and four classes (Thread, ThreadGroup, ThreadLocal, and InheritableThreadLocal) in the java.lang package. After exploring Runnable and Thread (and mentioning ThreadGroup during this exploration), this section explores thread synchronization, ThreadLocal, and InheritableThreadLocal. NOTE: Java version 5 introduced the java.util.concurrent package as a high-level alternative to the low-level Threading API. (I will discuss this package in Chapter 9.) Although java.util.concurrent is the preferred API for working with threads, you should also be somewhat familiar with Threading because it is helpful in simple threading scenarios. Also, you might have to analyze someone else’s source code that depends on Threading. CHAPTER 7: Exploring the Basic APIs Part 2 288 Runnable and Thread Java provides the Runnable interface to identify those objects that supply code for threads to execute via this interface’s solitary public void run() method—a thread receives no arguments and returns no value. Classes implement Runnable to supply this code, and one of these classes is Thread. Thread provides a consistent interface to the underlying operating system’s threading architecture. (The operating system is typically responsible for creating and managing threads.) Thread makes it possible to associate code with threads, as well as start and manage those threads. Each Thread instance associates with a single thread. Thread declares several constructors for initializing Thread objects. Some of these constructors take Runnable arguments: you can supply code to run without having to extend Thread. Other constructors do not take Runnable arguments: you must extend Thread and override its run() method to supply the code to run. For example, Thread(Runnable runnable) initializes a new Thread object to the specified runnable whose code is to be executed. In contrast, Thread() does not initialize Thread to a Runnable argument. Instead, your Thread subclass provides a constructor that calls Thread(), and the subclass also overrides Thread’s run() method. In the absence of an explicit name argument, each constructor assigns a unique default name (starting with Thread-) to the Thread object. Names make it possible to differentiate threads. In contrast to the previous two constructors, which choose default names, Thread(String threadName) lets you specify your own thread name. Thread also declares methods for starting and managing threads. Table 7–5 describes many of the more useful methods. Table 7–5. Thread Methods Method Description static Thread currentThread() Return the Thread object associated with the thread that calls this method. String getName() Return the name associated with this Thread object. Thread.State getState() Return the state of the thread associated with this Thread object. The state is identified by the Thread.State enum as one of BLOCKED (waiting to acquire a lock, discussed later), NEW (created but not started), RUNNABLE (executing), TERMINATED (the thread has died), TIMED_WAITING (waiting for a specified amount of time to elapse), or WAITING (waiting indefinitely). void interrupt() Set the interrupt status flag in this Thread object. If the associated thread is blocked or waiting, clear this flag and wake up the thread by throwing an instance of the checked InterruptedException class. CHAPTER 7: Exploring the Basic APIs Part 2 289 Method Description static boolean interrupted() Return true when the thread associated with this Thread object has a pending interrupt request. Clear the interrupt status flag. boolean isAlive() Return true to indicate that this Thread object’s associated thread is alive and not dead. A thread’s lifespan ranges from just before it is actually started within the start() method to just after it leaves the run() method, at which point it dies. boolean isDaemon() Return true when the thread associated with this Thread object is a daemon thread, a thread that acts as a helper to a user thread (nondaemon thread) and dies automatically when the application’s last nondaemon thread dies so the application can exit. boolean isInterrupted() Return true when the thread associated with this Thread object has a pending interrupt request. void join() The thread that calls this method on this Thread object waits for the thread associated with this object to die. This method throws InterruptedException when this Thread object’s interrupt() method is called. void join(long millis) The thread that calls this method on this Thread object waits for the thread associated with this object to die, or until millis milliseconds have elapsed, whichever happens first. This method throws InterruptedException when this Thread object’s interrupt() method is called. void setDaemon(boolean isDaemon) Mark this Thread object’s associated thread as a daemon thread when isDaemon is true. This method throws java.lang.IllegalThreadStateException when the thread has not yet been created and started. void setName(String threadName) Assign threadName’s value to this Thread object as the name of its associated thread. static void sleep(long time) Pause the thread associated with this Thread object for time milliseconds. This method throws InterruptedException when this Thread object’s interrupt() method is called while the thread is sleeping. void start() Create and start this Thread object’s associated thread. This method throws IllegalThreadStateException when the thread was previously started and is running or has died. Listing 7–3 introduces you to the Threading API via a main() method that demonstrates Runnable, Thread(Runnable runnable), currentThread(), getName(), and start(). CHAPTER 7: Exploring the Basic APIs Part 2 290 Listing 7–3. A pair of counting threads public class CountingThreads { public static void main(String[] args) { Runnable r = new Runnable() { @Override public void run() { String name = Thread.currentThread().getName(); int count = 0; while (true) System.out.println(name + ": " + count++); } }; Thread thdA = new Thread(r); Thread thdB = new Thread(r); thdA.start(); thdB.start(); } } The default main thread that executes main() first instantiates an anonymous class that implements Runnable. It then creates two Thread objects, initializing each object to the runnable, and calls Thread’s start() method to create and start both threads. After completing these tasks, the main thread exits main() and dies. Each of the two started threads executes the runnable’s run() method. It calls Thread’s currentThread() method to obtain its associated Thread instance, uses this instance to call Thread’s getName() method to return its name, initializes count to 0, and enters an infinite loop where it outputs name and count and increments count on each iteration. TIP: To stop an application that does not end, press the Ctrl and C keys simultaneously. I observe both threads alternating in their execution when I run this application on the Windows XP platform. Partial output from one run appears here: Thread-0: 0 Thread-0: 1 Thread-0: 2 Thread-0: 3 Thread-0: 4 Thread-0: 5 Thread-0: 6 Thread-0: 7 Thread-1: 0 Thread-1: 1 Thread-1: 2 Thread-1: 3 Thread-1: 4 Thread-1: 5 Thread-1: 6 CHAPTER 7: Exploring the Basic APIs Part 2 291 Thread-1: 7 Thread-1: 8 Thread-1: 9 Thread-1: 10 Thread-1: 11 Thread-1: 12 Thread-1: 13 Thread-1: 14 Thread-1: 15 Thread-0: 8 Thread-0: 9 When a computer has enough processors and/or processor cores, the computer’s operating system assigns a separate thread to each processor or core so the threads execute concurrently (at the same time). When a computer does not have enough processors and/or cores, a thread must wait its turn to use the shared processor/core. The operating system uses a scheduler (http://en.wikipedia.org/wiki/Scheduling_%28computing%29) to determine when a waiting thread executes. The following list identifies three different schedulers:  Linux 2.6 through 2.6.22 uses the O(1) scheduler (http://en.wikipedia.org/wiki/O%281%29_scheduler).  Linux 2.6.23 uses the Completely Fair Scheduler (http://en.wikipedia.org/wiki/Completely_Fair_Scheduler).  Windows NT-based operating systems (NT, XP, and Vista) use a multilevel feedback queue scheduler (http://en.wikipedia.org/wiki/Multilevel_feedback_queue). The previous output from the counting threads application resulted from running this application via Windows XP’s multilevel feedback queue scheduler. Because of this scheduler, both threads take turns executing. CAUTION: Although this output indicates that the first thread starts executing, never assume that the thread associated with the Thread object whose start() method is called first is the first thread to execute. While this might be true of some schedulers, it might not be true of others. A multilevel feedback queue and many other thread schedulers take the concept of priority (thread relative importance) into account. They often combine preemptive scheduling (higher priority threads preempt—interrupt and run instead of—lower priority threads) with round robin scheduling (equal priority threads are given equal slices of time, which are known as time slices, and take turns executing). Thread supports priority via its void setPriority(int priority) method (set the priority of this Thread object’s thread to priority, which ranges from Thread.MIN_PRIORITY to Thread.MAX_PRIORITY—Thread.NORMAL_PRIORITY identifies the default priority) and int getPriority() method (return the current priority). CHAPTER 7: Exploring the Basic APIs Part 2 292 CAUTION: Using the setPriority() method can impact an application’s portability across platforms because different schedulers can handle a priority change in different ways. For example, one platform’s scheduler might delay lower priority threads from executing until higher priority threads finish. This delaying can lead to indefinite postponement or starvation because lower priority threads “starve” while waiting indefinitely for their turn to execute, and this can seriously hurt the application’s performance. Another platform’s scheduler might not indefinitely delay lower priority threads, improving application performance. Listing 7–4 refactors Listing 7–3’s main() method to give each thread a nondefault name, and to put each thread to sleep after outputting name and count. Listing 7–4. A pair of counting threads revisited public static void main(String[] args) { Runnable r = new Runnable() { @Override public void run() { String name = Thread.currentThread().getName(); int count = 0; while (true) { System.out.println(name + ": " + count++); try { Thread.sleep(100); } catch (InterruptedException ie) { } } } }; Thread thdA = new Thread(r); thdA.setName("A"); Thread thdB = new Thread(r); thdB.setName("B"); thdA.start(); thdB.start(); } Threads A and B execute Thread.sleep(100); to sleep for 100 milliseconds. This sleep results in each thread executing more frequently, as the following partial output reveals: A: 0 B: 0 A: 1 B: 1 A: 2 B: 2 A: 3 CHAPTER 7: Exploring the Basic APIs Part 2 293 B: 3 A: 4 B: 4 A: 5 B: 5 A thread will occasionally start another thread to perform a lengthy calculation, download a large file, or perform some other time-consuming activity. After finishing its other tasks, the thread that started the worker thread is ready to process the results of the worker thread and waits for the worker thread to finish and die. It is possible to wait for the worker thread to die by using a while loop that repeatedly calls Thread’s isAlive() method on the worker thread’s Thread object and sleeps for a certain length of time when this method returns true. However, Listing 7–5 demonstrates a less verbose alternative: the join() method. Listing 7–5. Joining the default main thread with a background thread public static void main(String[] args) { Runnable r = new Runnable() { @Override public void run() { System.out.println("Worker thread is simulating " + "work by sleeping for 5 seconds."); try { Thread.sleep(5000); } catch (InterruptedException ie) { } System.out.println("Worker thread is dying"); } }; Thread thd = new Thread(r); thd.start(); System.out.println("Default main thread is doing work."); try { Thread.sleep(2000); } catch (InterruptedException ie) { } System.out.println("Default main thread has finished its work."); System.out.println("Default main thread is waiting for worker thread " + "to die."); try { thd.join(); } catch (InterruptedException ie) { } CHAPTER 7: Exploring the Basic APIs Part 2 294 System.out.println("Main thread is dying"); } This listing demonstrates the default main thread starting a worker thread, performing some work, and then waiting for the worker thread to die by calling join() via the worker thread’s thd object. When you run this application, you will discover output similar to the following (message order might differ somewhat): Default main thread is doing work. Worker thread is simulating work by sleeping for 5 seconds. Default main thread has finished its work. Default main thread is waiting for worker thread to die. Worker thread is dying Main thread is dying Every Thread object belongs to some ThreadGroup object; Thread declares a ThreadGroup getThreadGroup() method that returns this object. You should ignore thread groups because they are not that useful. If you need to logically group Thread objects, you should use an array or collection instead. CAUTION: Various ThreadGroup methods are flawed. For example, int enumerate(Thread[] threads) will not include all active threads in its enumeration when its threads array argument is too small to store their Thread objects. Although you might think that you could use the return value from the int activeCount() method to properly size this array, there is no guarantee that the array will be large enough because activeCount()’s return value fluctuates with the creation and death of threads. However, you should still know about ThreadGroup because of its contribution in handling exceptions that are thrown while a thread is executing. Listing 7–6 sets the stage for learning about exception handling by presenting a run() method that attempts to divide an integer by 0, which results in a thrown ArithmeticException instance. Listing 7–6. Throwing an exception from the run() method public static void main(String[] args) { Runnable r = new Runnable() { @Override public void run() { int x = 1/0; // Line 8 } }; Thread thd = new Thread(r); thd.start(); } Run this application and you will see an exception trace that identifies the thrown ArithmeticException: Exception in thread "Thread-0" java.lang.ArithmeticException: / by zero This book was purchased by harshamv@variable3.comCHAPTER 7: Exploring the Basic APIs Part 2 295 at ExceptionThread$1.run(ExceptionThread.java:8) at java.lang.Thread.run(Unknown Source) When an exception is thrown out of the run() method, the thread terminates and the following activities take place:  The virtual machine looks for an instance of Thread.UncaughtExceptionHandler installed via Thread’s void setUncaughtExceptionHandler(Thread.UncaughtExceptionHandler eh) method. When this handler is found, it passes execution to the instance’s void uncaughtException(Thread t, Throwable e) method, where t identifies the Thread object of the thread that threw the exception, and e identifies the thrown exception or error—perhaps an OutOfMemoryError instance was thrown. If this method throws an exception/error, the exception/error is ignored by the virtual machine.  Assuming that setUncaughtExceptionHandler() was not called to install a handler, the virtual machine passes control to the associated ThreadGroup object’s uncaughtException(Thread t, Throwable e) method. Assuming that ThreadGroup was not extended, and that its uncaughtException() method was not overridden to handle the exception, uncaughtException() passes control to the parent ThreadGroup object’s uncaughtException() method when a parent ThreadGroup is present. Otherwise, it checks to see if a default uncaught exception handler has been installed (via Thread’s static void setDefaultUncaughtExceptionHandler (Thread.UncaughtExceptionHandler handler) method.) If a default uncaught exception handler has been installed, its uncaughtException() method is called with the same two arguments. Otherwise, uncaughtException() checks its Throwable argument to determine if it is an instance of java.lang.ThreadDeath. If so, nothing special is done. Otherwise, as Listing 7–6’s exception message shows, a message containing the thread’s name, as returned from the thread’s getName() method, and a stack backtrace, using the Throwable argument’s printStackTrace() method, is printed to the standard error stream. Listing 7–7 demonstrates Thread’s setUncaughtExceptionHandler() and setDefaultUncaughtExceptionHandler() methods. Listing 7–7. Demonstrating uncaught exception handlers public static void main(String[] args) { Runnable r = new Runnable() { @Override public void run() { int x = 1/0; } }; CHAPTER 7: Exploring the Basic APIs Part 2 296 Thread thd = new Thread(r); Thread.UncaughtExceptionHandler uceh; uceh = new Thread.UncaughtExceptionHandler() { public void uncaughtException(Thread t, Throwable e) { System.out.println("Caught throwable " + e + " for thread " + t); } }; thd.setUncaughtExceptionHandler(uceh); uceh = new Thread.UncaughtExceptionHandler() { public void uncaughtException(Thread t, Throwable e) { System.out.println("Default uncaught exception handler"); System.out.println("Caught throwable " + e + " for thread " + t); } }; thd.setDefaultUncaughtExceptionHandler(uceh); thd.start(); } When you run this application, you will observe the following output: Caught throwable java.lang.ArithmeticException: / by zero for thread Thread[Thread-0,5,main] You will not also see the default uncaught exception handler’s output because the default handler is not called. To see that output, you must comment out thd.setUncaughtExceptionHandler(uceh);. If you also comment out thd.setDefaultUncaughtExceptionHandler(uceh);, you will see Listing 7–6’s output. CAUTION: Thread declares several deprecated methods, including stop() (stop an executing thread). These methods have been deprecated because they are unsafe. Do not use these deprecated methods. (I will show you how to safely stop a thread later in this chapter.) Also, you should avoid the static void yield() method, which is intended to switch execution from the current thread to another thread, because it can affect portability and hurt application performance. Although yield() might switch to another thread on some platforms (which can improve performance), yield() might only return to the current thread on other platforms (which hurts performance because the yield() call has only wasted time). Thread Synchronization Throughout its execution, each thread is isolated from other threads because it has been given its own method-call stack. However, threads can still interfere with each other when they access and manipulate shared data. This interference can corrupt the shared data, and this corruption can cause an application to fail. CHAPTER 7: Exploring the Basic APIs Part 2 297 For example, consider a checking account in which a husband and wife have joint access. Suppose that the husband and wife decide to empty this account at the same time without knowing that the other is doing the same thing. Listing 7–8 demonstrates this scenario. Listing 7–8. A problematic checking account public class CheckingAccount { private int balance; public CheckingAccount(int initialBalance) { balance = initialBalance; } public boolean withdraw(int amount) { if (amount <= balance) { try { Thread.sleep((int)(Math.random()*200)); } catch (InterruptedException ie) { } balance -= amount; return true; } return false; } public static void main(String[] args) { final CheckingAccount ca = new CheckingAccount(100); Runnable r = new Runnable() { public void run() { String name = Thread.currentThread().getName(); for (int i = 0; i < 10; i++) System.out.println (name + " withdraws $10: " + ca.withdraw(10)); } }; Thread thdHusband = new Thread(r); thdHusband.setName("Husband"); Thread thdWife = new Thread(r); thdWife.setName("Wife"); thdHusband.start(); thdWife.start(); } } This application lets more money be withdrawn than is available in the account. For example, the following output reveals $110 being withdrawn when only $100 is available: Wife withdraws $10: true CHAPTER 7: Exploring the Basic APIs Part 2 298 Wife withdraws $10: true Husband withdraws $10: true Wife withdraws $10: true Husband withdraws $10: true Wife withdraws $10: true Husband withdraws $10: true Husband withdraws $10: true Husband withdraws $10: true Husband withdraws $10: true Husband withdraws $10: false Husband withdraws $10: false Husband withdraws $10: false Husband withdraws $10: false Wife withdraws $10: true Wife withdraws $10: false Wife withdraws $10: false Wife withdraws $10: false Wife withdraws $10: false Wife withdraws $10: false The reason why more money is withdrawn than is available for withdrawal is that a race condition exists between the husband and wife threads. NOTE: A race condition is a scenario in which multiple threads update the same object at the same time or nearly at the same time. Part of the object stores values written to it by one thread, and another part of the object stores values written to it by another thread. The race condition exists because the actions of checking the amount for withdrawal to ensure that it is less than what appears in the balance and deducting the amount from the balance are not atomic (indivisible) operations. (Although atoms are divisible, atomic is commonly used to refer to something being indivisible.) NOTE: The Thread.sleep() method call that sleeps for a variable amount of time (up to a maximum of 199 milliseconds) is present so that you can observe more money being withdrawn than is available for withdrawal. Without this method call, you might have to execute the application hundreds of times (or more) to witness this problem, because the scheduler might rarely pause a thread between the amount <= balance expression and the balance -= amount; expression statement—the code executes rapidly. Consider the following scenario:  The Husband thread executes withdraw()’s amount <= balance expression, which returns true. The scheduler pauses the Husband thread and lets the Wife thread execute.  The Wife thread executes withdraw()’s amount <= balance expression, which returns true. CHAPTER 7: Exploring the Basic APIs Part 2 299  The Wife thread performs the withdrawal. The scheduler pauses the Wife thread and lets the Husband thread execute.  The Husband thread performs the withdrawal. This problem can be corrected by synchronizing access to withdraw() so that only one thread at a time can execute inside this method. You synchronize access at the method level by adding reserved word synchronized to the method header prior to the method’s return type; for example, public synchronized boolean withdraw(int amount). As I demonstrate later, you can also synchronize access to a block of statements by specifying synchronized(object) { /* synchronized statements */ }, where object is an arbitrary object reference. No thread can enter a synchronized method or block until execution leaves the method/block; this is known as mutual exclusion. Synchronization is implemented in terms of monitors and locks. A monitor is a concurrency construct for controlling access to a critical section, a region of code that must execute atomically. It is identified at the source code level as a synchronized method or a synchronized block. A lock is a token that a thread must acquire before a monitor allows that thread to execute inside a monitor’s critical section. The token is released automatically when the thread exits the monitor, to give another thread an opportunity to acquire the token and enter the monitor. NOTE: A thread that has acquired a lock does not release this lock when it calls one of Thread’s sleep() methods. A thread entering a synchronized instance method acquires the lock associated with the object on which the method is called. A thread entering a synchronized class method acquires the lock associated with the class’s Class object. Finally, a thread entering a synchronized block acquires the lock associated with the block’s controlling object. TIP: Thread declares a public static boolean holdsLock(Object o) method that returns true when the calling thread holds the monitor lock on object o. You will find this method handy in assertion statements, such as assert Thread.holdsLock(o);. The need for synchronization is often subtle. For example, Listing 7–9’s ID utility class declares a getNextID() method that returns a unique long-based ID, perhaps to be used when generating unique filenames. Although you might not think so, this method can cause data corruption and return duplicate values. Listing 7–9. A utility class for returning unique IDs public class ID { private static long nextID = 0; public static long getNextID() CHAPTER 7: Exploring the Basic APIs Part 2 300 { return nextID++; } } There are two lack-of-synchronization problems with getNextID(). Because 32-bit virtual machine implementations require two steps to update a 64-bit long integer, adding 1 to nextID is not atomic: the scheduler could interrupt a thread that has only updated half of nextID, which corrupts the contents of this variable. NOTE: Variables of type long and double are subject to corruption when being written to in an unsynchronized context on 32-bit virtual machines. This problem does not occur with variables of type boolean, byte, char, float, int, or short; each type occupies 32 bits or less. Assume that multiple threads call getNextID(). Because postincrement (++) reads and writes the nextID field in two steps, multiple threads might retrieve the same value. For example, thread A executes ++, reading nextID but not incrementing its value before being interrupted by the scheduler. Thread B now executes and reads the same value. Both problems can be corrected by synchronizing access to nextID so that only one thread can execute this method’s code. All that is required is to add synchronized to the method header prior to the method’s return type; for example, public static synchronized int getNextID(). Synchronization is also used to communicate between threads. For example, you might design your own mechanism for stopping a thread (because you cannot use Thread’s unsafe stop() methods for this task). Listing 7–10 shows how you might accomplish this task. Listing 7–10. Attempting to stop a thread public static void main(String[] args) { class StoppableThread extends Thread { private boolean stopped = false; @Override public void run() { while(!stopped) System.out.println("running"); } public void stopThread() { stopped = true; } } StoppableThread thd = new StoppableThread(); thd.start(); try { Thread.sleep(1000); // sleep for 1 second CHAPTER 7: Exploring the Basic APIs Part 2 301 } catch (InterruptedException ie) { } thd.stopThread(); } Listing 7–10 introduces a main() method with a local class named StoppableThread that subclasses Thread. StoppableThread declares a stopped field initialized to false, a stopThread() method that sets this field to true, and a run() method whose infinite loop checks stopped on each loop iteration to see if its value has changed to true. After instantiating StoppableThread, the default main thread starts the thread associated with this Thread object. It then sleeps for one second and calls StoppableThread’s stop() method before dying. When you run this application on a single- processor/single-core machine, you will probably observe the application stopping. You might not see this stoppage when the application runs on a multiprocessor machine or a uniprocessor machine with multiple cores. For performance reasons, each processor or core probably has its own cache with its own copy of stopped. When one thread modifies its copy of this field, the other thread’s copy of stopped is not changed. Listing 7–11 refactors Listing 7–10 to guarantee that the application will run correctly on all kinds of machines. Listing 7–11. Guaranteed stoppage on a multiprocessor/multicore machine public static void main(String[] args) { class StoppableThread extends Thread { private boolean stopped = false; @Override public void run() { while(!isStopped()) System.out.println("running"); } public synchronized void stopThread() { stopped = true; } private synchronized boolean isStopped() { return stopped; } } StoppableThread thd = new StoppableThread(); thd.start(); try { Thread.sleep(1000); // sleep for 1 second } catch (InterruptedException ie) { } CHAPTER 7: Exploring the Basic APIs Part 2 302 thd.stopThread(); } Listing 7–11’s stopThread() and isStopped() methods are synchronized to support thread communication (between the default main thread that calls stopThread() and the started thread that executes inside run()). When a thread enters one of these methods, it is guaranteed to access a single shared copy of the stopped field (not a cached copy). Synchronization is necessary to support mutual exclusion or mutual exclusion combined with thread communication. However, there exists an alternative to synchronization when the only purpose is to communicate between threads. This alternative is reserved word volatile, which Listing 7–12 demonstrates. Listing 7–12. The volatile alternative to synchronization for thread communication public static void main(String[] args) { class StoppableThread extends Thread { private volatile boolean stopped = false; @Override public void run() { while(!stopped) System.out.println("running"); } public void stopThread() { stopped = true; } } StoppableThread thd = new StoppableThread(); thd.start(); try { Thread.sleep(1000); // sleep for 1 second } catch (InterruptedException ie) { } thd.stopThread(); } Listing 7–12 declares stopped to be volatile; threads that access this field will always access a single shared copy (not cached copies on multiprocessor/multicore machines). In addition to generating code that is less verbose, volatile might offer improved performance over synchronization. CHAPTER 7: Exploring the Basic APIs Part 2 303 CAUTION: You should only use volatile in the context of thread communication. Also, you can only use this reserved word in the context of field declarations. Although you can declare double and long fields volatile, you should avoid doing so on 32-bit virtual machines because it takes two operations to access a double or long variable’s value, and mutual exclusion via synchronization is required to access their values safely. Object’s wait(), notify(), and notifyAll() methods support a form of thread communication where a thread voluntarily waits for some condition (a prerequisite for continued execution) to arise, at which time another thread notifies the waiting thread that it can continue. wait() causes its calling thread to wait on an object’s monitor, and notify() and notifyAll() wake up one or all threads waiting on the monitor. CAUTION: Because the wait(), notify(), and notifyAll() methods depend on a lock, they cannot be called from outside of a synchronized method or synchronized block. If you fail to heed this warning, you will encounter a thrown instance of the java.lang.IllegalMonitorStateException class. Also, a thread that has acquired a lock releases this lock when it calls one of Object’s wait() methods. A classic example of thread communication involving conditions is the relationship between a producer thread and a consumer thread. The producer thread produces data items to be consumed by the consumer thread. Each produced data item is stored in a shared variable. Imagine that the threads are not communicating and are running at different speeds. The producer might produce a new data item and record it in the shared variable before the consumer retrieves the previous data item for processing. Also, the consumer might retrieve the contents of the shared variable before a new data item is produced. To overcome those problems, the producer thread must wait until it is notified that the previously produced data item has been consumed, and the consumer thread must wait until it is notified that a new data item has been produced. Listing 7–13 shows you how to accomplish this task via wait() and notify(). Listing 7–13. The producer-consumer relationship public class PC { public static void main(String[] args) { Shared s = new Shared(); new Producer(s).start(); new Consumer(s).start(); } } class Shared { private char c = '\u0000'; CHAPTER 7: Exploring the Basic APIs Part 2 304 private boolean writeable = true; synchronized void setSharedChar(char c) { while (!writeable) try { wait(); } catch (InterruptedException e) {} this.c = c; writeable = false; notify(); } synchronized char getSharedChar() { while (writeable) try { wait(); } catch (InterruptedException e) {} writeable = true; notify(); return c; } } class Producer extends Thread { private Shared s; Producer(Shared s) { this.s = s; } @Override public void run() { for (char ch = 'A'; ch <= 'Z'; ch++) { synchronized(s) { s.setSharedChar(ch); System.out.println(ch + " produced by producer."); } } } } class Consumer extends Thread { private Shared s; Consumer(Shared s) { this.s = s; } @Override public void run() { char ch; This book was purchased by harshamv@variable3.comCHAPTER 7: Exploring the Basic APIs Part 2 305 do { synchronized(s) { ch = s.getSharedChar(); System.out.println(ch + " consumed by consumer."); } } while (ch != 'Z'); } } The application creates a Shared object and two threads that get a copy of the object’s reference. The producer calls the object’s setSharedChar() method to save each of 26 uppercase letters; the consumer calls the object’s getSharedChar() method to acquire each letter. The writeable instance field tracks two conditions: the producer waiting on the consumer to consume a data item, and the consumer waiting on the producer to produce a new data item. It helps coordinate execution of the producer and consumer. The following scenario, where the consumer executes first, illustrates this coordination: 1. The consumer executes s.getSharedChar() to retrieve a letter. 2. Inside of that synchronized method, the consumer calls wait() because writeable contains true. The consumer now waits until it receives notification from the producer. 3. The producer eventually executes s.setSharedChar(ch);. 4. When the producer enters that synchronized method (which is possible because the consumer released the lock inside of the wait() method prior to waiting), the producer discovers writeable’s value to be true and does not call wait(). 5. The producer saves the character, sets writeable to false (which will cause the producer to wait on the next setSharedChar() call when the consumer has not consumed the character by that time), and calls notify() to awaken the consumer (assuming the consumer is waiting). 6. The producer exits setSharedChar(char c). 7. The consumer wakes up (and reacquires the lock), sets writeable to true (which will cause the consumer to wait on the next getSharedChar() call when the producer has not produced a character by that time), notifies the producer to awaken that thread (assuming the producer is waiting), and returns the shared character. Although the synchronization works correctly, you might observe output (on some platforms) that shows multiple producing messages before a consuming message. For example, you might see A produced by producer., followed by B produced by CHAPTER 7: Exploring the Basic APIs Part 2 306 producer., followed by A consumed by consumer., at the beginning of the application’s output. This strange output order is caused by the call to setSharedChar() followed by its companion System.out.println() method call not being atomic, and by the call to getSharedChar() followed by its companion System.out.println() method call not being atomic. The output order is corrected by wrapping each of these method call pairs in a synchronized block that synchronizes on the Shared object referenced by s. When you run this application, its output should always appear in the same alternating order, as shown next (only the first few lines are shown for brevity): A produced by producer. A consumed by consumer. B produced by producer. B consumed by consumer. C produced by producer. C consumed by consumer. D produced by producer. D consumed by consumer. CAUTION: Never call wait() outside of a loop. The loop tests the condition (!writeable or writeable in the previous example) before and after the wait() call. Testing the condition before calling wait() ensures liveness. If this test was not present, and if the condition held and notify() had been called prior to wait() being called, it is unlikely that the waiting thread would ever wake up. Retesting the condition after calling wait() ensures safety. If retesting did not occur, and if the condition did not hold after the thread had awakened from the wait() call (perhaps another thread called notify() accidentally when the condition did not hold), the thread would proceed to destroy the lock’s protected invariants. Too much synchronization can be problematic. If you are not careful, you might encounter a situation where locks are acquired by multiple threads, neither thread holds its own lock but holds the lock needed by some other thread, and neither thread can enter and later exit its critical section to release its held lock because some other thread holds the lock to that critical section. Listing 7–14’s atypical example demonstrates this scenario, which is known as deadlock. Listing 7–14. A pathological case of deadlock public class Deadlock { private Object lock1 = new Object(); private Object lock2 = new Object(); public void instanceMethod1() { synchronized(lock1) { synchronized(lock2) { System.out.println("first thread in instanceMethod1"); // critical section guarded first by CHAPTER 7: Exploring the Basic APIs Part 2 307 // lock1 and then by lock2 } } } public void instanceMethod2() { synchronized(lock2) { synchronized(lock1) { System.out.println("second thread in instanceMethod2"); // critical section guarded first by // lock2 and then by lock1 } } } public static void main(String[] args) { final Deadlock dl = new Deadlock(); Runnable r1 = new Runnable() { @Override public void run() { while(true) dl.instanceMethod1(); } }; Thread thdA = new Thread(r1); Runnable r2 = new Runnable() { @Override public void run() { while(true) dl.instanceMethod2(); } }; Thread thdB = new Thread(r2); thdA.start(); thdB.start(); } } Listing 7–14’s thread A and thread B call instanceMethod1() and instanceMethod2(), respectively, at different times. Consider the following execution sequence: 1. Thread A calls instanceMethod1(), obtains the lock assigned to the lock1-referenced object, and enters its outer critical section (but has not yet acquired the lock assigned to the lock2-referenced object). 2. Thread B calls instanceMethod2(), obtains the lock assigned to the lock2-referenced object, and enters its outer critical section (but has not yet acquired the lock assigned to the lock1-referenced object). CHAPTER 7: Exploring the Basic APIs Part 2 308 3. Thread A attempts to acquire the lock associated with lock2. The virtual machine forces the thread to wait outside of the inner critical section because thread B holds that lock. 4. Thread B attempts to acquire the lock associated with lock1. The virtual machine forces the thread to wait outside of the inner critical section because thread A holds that lock. 5. Neither thread can proceed because the other thread holds the needed lock. We have a deadlock situation and the program (at least in the context of the two threads) freezes up. Although the previous example clearly identifies a deadlock state, it is often not that easy to detect deadlock. For example, your code might contain the following circular relationship among various classes (in several source files):  Class A’s synchronized method calls class B’s synchronized method.  Class B’s synchronized method calls class C’s synchronized method.  Class C’s synchronized method calls class A’s synchronized method. If thread A calls class A’s synchronized method and thread B calls class C’s synchronized method, thread B will block when it attempts to call class A’s synchronized method and thread A is still inside of that method. Thread A will continue to execute until it calls class C’s synchronized method, and then block. Deadlock results. NOTE: Neither the Java language nor the virtual machine provides a way to prevent deadlock, and so the burden falls on you. The simplest way to prevent deadlock from happening is to avoid having either a synchronized method or a synchronized block call another synchronized method/block. Although this advice prevents deadlock from happening, it is impractical because one of your synchronized methods/blocks might need to call a synchronized method in a Java API, and the advice is overkill because the synchronized method/block being called might not call any other synchronized method/block, so deadlock would not occur. You will sometimes want to associate per-thread data (such a user ID) with a thread. Although you can accomplish this task with a local variable, you can only do so while the local variable exists. You could use an instance field to keep this data around longer, but then you would have to deal with synchronization. Thankfully, Java supplies ThreadLocal as a simple alternative. Each instance of the ThreadLocal class describes a thread-local variable, which is a variable that provides a separate storage slot to each thread that accesses the variable. You can think of a thread-local variable as a multislot variable in which each thread can store a different value in the same variable. Each thread sees only its value and is unaware of other threads having their own values in this variable. CHAPTER 7: Exploring the Basic APIs Part 2 309 ThreadLocal is generically declared as ThreadLocal, where T identifies the type of value that is stored in the variable. This class declares the following constructor and methods:  ThreadLocal() creates a new thread-local variable.  T get() returns the value in the calling thread’s storage slot. If an entry does not exist when the thread calls this method, get() calls initialValue().  T initialValue() creates the calling thread’s storage slot and stores an initial (default) value in this slot. The initial value defaults to null. You must subclass ThreadLocal and override this protected method to provide a more suitable initial value.  void remove() removes the calling thread’s storage slot. If this method is followed by get() with no intervening set(), get() calls initialValue().  void set(T value) sets the value of the calling thread’s storage slot to value. Listing 7–15 shows you how to use ThreadLocal to associate a different user ID with each of two threads. Listing 7–15. Different user IDs for different threads private static volatile ThreadLocal userID = new ThreadLocal(); public static void main(String[] args) { Runnable r = new Runnable() { @Override public void run() { String name = Thread.currentThread().getName(); if (name.equals("A")) userID.set("foxtrot"); else userID.set("charlie"); System.out.println(name + " " + userID.get()); } }; Thread thdA = new Thread(r); thdA.setName("A"); Thread thdB = new Thread(r); thdB.setName("B"); thdA.start(); thdB.start(); } After instantiating ThreadLocal and assigning the reference to a volatile class field named userID (the field is volatile because it is accessed by different threads, which CHAPTER 7: Exploring the Basic APIs Part 2 310 might execute on a multiprocessor/multicore machine), the default main thread creates two more threads that store different String objects in userID and output their objects. When you run this application, you will observe the following output (possibly not in this order): A foxtrot B charlie Values stored in thread-local variables are not related. When a new thread is created, it gets a new storage slot containing initialValue()’s value. Perhaps you would prefer to pass a value from a parent thread, a thread that creates another thread, to a child thread, the created thread. You accomplish this task with InheritableThreadLocal. InheritableThreadLocal is a subclass of ThreadLocal. In addition to declaring a public InheritableThreadLocal() constructor, this class declares the following protected method:  T childValue(T parentValue) calculates the child’s initial value as a function of the parent’s value at the time the child thread is created. This method is called from the parent thread before the child thread is started. The method returns the argument passed to parentValue and should be overridden when another value is desired. Listing 7–16 shows you how to use InheritableThreadLocal to pass a parent thread’s Integer object to a child thread. Listing 7–16. Passing an object from parent thread to child thread private static volatile InheritableThreadLocal intVal = new InheritableThreadLocal(); public static void main(String[] args) { Runnable rP = new Runnable() { @Override public void run() { intVal.set(new Integer(10)); Runnable rC = new Runnable() { public void run() { Thread thd; thd = Thread.currentThread(); String name = thd.getName(); System.out.println(name + " " + intVal.get()); } }; Thread thdChild = new Thread(rC); thdChild.setName("Child"); thdChild.start(); } }; new Thread(rP).start(); } CHAPTER 7: Exploring the Basic APIs Part 2 311 After instantiating InheritableThreadLocal and assigning it to a volatile class field named intVal, the default main thread creates a parent thread, which stores an Integer object containing 10 in intVal. The parent thread creates a child thread, which accesses intVal and retrieves its parent thread’s Integer object. When you run this application, you will observe the following output: Child 10 EXERCISES The following exercises are designed to test your understanding of this chapter’s additional basic APIs: 1. What is reflection? 2. What is the difference between Class’s getDeclaredFields() and getFields() methods? 3. How would you determine if the method represented by a Method object is abstract? 4. Identify the three ways of obtaining a Class object. 5. True or false: A string literal is a String object. 6. What is the purpose of String’s intern() method? 7. How do String and StringBuffer differ? 8. How do StringBuffer and StringBuilder differ? 9. What does System’s arraycopy() method accomplish? 10. What is a thread? 11. What is the purpose of the Runnable interface? 12. What is the purpose of the Thread class? 13. True or false: A Thread object associates with multiple threads. 14. What is a race condition? 15. What is synchronization? 16. How is synchronization implemented? 17. How does synchronization work? 18. True or false: Variables of type long or double are not atomic on 32-bit virtual machines. 19. What is the purpose of reserved word volatile? 20. True or false: Object’s wait() methods can be called from outside of a synchronized method or block. 21. What is deadlock? 22. What is the purpose of the ThreadLocal class? CHAPTER 7: Exploring the Basic APIs Part 2 312 23. How does InheritableThreadLocal differ from ThreadLocal? 24. In Chapter 6, Listing 6-14’s demonstration of the SoftReference class includes the following array declaration and inefficient loop: String[] imageNames = new String[NUM_IMAGES]; for (int i = 0; i < imageNames.length; i++) imageNames[i] = new String("image" + i + ".gif"); Rewrite this loop to use StringBuffer. 25. Class declares boolean isAnnotation(), boolean isEnum(), and boolean isInterface() methods that return true when the Class object represents an annotation type, an enum, or an interface, respectively. Create a Classify application that uses Class’s forName() method to load its single command-line argument, which will represent an annotation type, enum, interface, or class (the default). Use a chained if-else statement along with the aforementioned methods to output Annotation, Enum, Interface, or Class. 26. The output from Listing 7–1’s ExploreType application does not look like a class declaration for the Boolean class. Improve this application so that java ExploreType java.lang.Boolean generates the following output: public class java.lang.Boolean { // FIELDS public static final java.lang.Boolean java.lang.Boolean.TRUE public static final java.lang.Boolean java.lang.Boolean.FALSE public static final java.lang.Class java.lang.Boolean.TYPE private final boolean java.lang.Boolean.value private static final long java.lang.Boolean.serialVersionUID // CONSTRUCTORS public java.lang.Boolean(java.lang.String) public java.lang.Boolean(boolean) // METHODS public int java.lang.Boolean.hashCode() public boolean java.lang.Boolean.equals(java.lang.Object) public int java.lang.Boolean.compareTo(java.lang.Boolean) public int java.lang.Boolean.compareTo(java.lang.Object) public static boolean java.lang.Boolean.getBoolean(java.lang.String) public static java.lang.String java.lang.Boolean.toString(boolean) public java.lang.String java.lang.Boolean.toString() public static java.lang.Boolean java.lang.Boolean.valueOf(java.lang.String) public static java.lang.Boolean java.lang.Boolean.valueOf(boolean) public boolean java.lang.Boolean.booleanValue() public static boolean java.lang.Boolean.parseBoolean(java.lang.String) private static boolean java.lang.Boolean.toBoolean(java.lang.String) } 27. Modify Listing 7–3’s CountingThreads application by marking the two started threads as daemon threads. What happens when you run the resulting application? CHAPTER 7: Exploring the Basic APIs Part 2 313 28. Modify Listing 7–3’s CountingThreads application by adding logic to stop both counting threads when the user presses the Enter key. The default main thread should call System.in.read() prior to terminating, and assign true to a variable named stopped after this method call returns. Each counting thread should test this variable to see if it contains true at the start of each loop iteration, and only continue the loop when the variable contains false. Summary The Reflection API lets applications learn about loaded classes, interfaces, enums, and annotation types. The API also lets applications instantiate classes, call methods, access fields, and perform other tasks reflectively. The entry point into the Reflection API is a special java.lang class named Class. Additional classes are located in the java.lang.reflect package, and include Constructor, Field, Method, AccessibleObject, and Array. The java.lang.String class represents a string as a sequence of characters. Because instances of this class are immutable, Java provides java.lang.StringBuffer for building a string more efficiently. The java.lang.System class provides access to standard input, standard output, and standard error, and other system-oriented resources. For example, System provides the arraycopy() method as the fastest portable way to copy one array to another. Finally, Java supports threads via its Threading API. This API consists of one interface (Runnable) and four classes (Thread, ThreadGroup, ThreadLocal, and InheritableThreadLocal) in the java.lang package. This chapter completes my coverage of Java’s basic APIs. Chapter 8 continues to explore Java’s foundational APIs by focusing on its utility APIs. Specifically, Chapter 8 introduces you to the collections framework. CHAPTER 7: Exploring the Basic APIs Part 2 314 This book was purchased by harshamv@variable3.com 315 315 Chapter Discovering the Collections Framework Java’s standard class library includes various utility APIs. An important category of utility APIs is the collections framework, which lets applications manage groups of objects. After presenting an overview of this framework, this chapter introduces you to the framework’s core interfaces, implementation classes, and utility classes. Chapter 8 ends by discussing Java’s classic collections classes. NOTE: Unless otherwise noted, Chapter 8 explores classes and interfaces that are located in the java.util package. Framework Overview A collection is a group of objects that are stored in an instance of a class designed for this purpose. You could create your own collections classes, but why should you waste time “reinventing the wheel” when you could spend that time focusing on your applications? For this and other reasons, Java provides the collections framework, which is a standardized architecture for representing and manipulating collections. The collections framework largely consists of three components:  Core interfaces: The framework employs core interfaces for manipulating collections independently of their implementations.  Implementation classes: The framework employs classes that provide different implementations of the core interfaces to address performance and other requirements.  Utility classes: The framework provides utility classes whose methods let you sort arrays, obtain synchronized collections, and more. 8 CHAPTER 8: Discovering the Collections Framework 316 The collections framework’s core interfaces include Iterable, Collection, List, Set, SortedSet, Queue, Map, and SortedMap. Collection extends Iterable; List, Set, and Queue each extend Collection; SortedSet extends Set; and SortedMap extends Map. Figure 8–1 illustrates the core interfaces hierarchy (arrows point to parent interfaces). Figure 8–1. A hierarchy of core interfaces The framework’s implementation classes include ArrayList, LinkedList, TreeSet, HashSet, LinkedHashSet, EnumSet, PriorityQueue, TreeMap, HashMap, LinkedHashMap, IdentityHashMap, WeakHashMap, and EnumMap. The name of each concrete class ends in a core interface name, identifying the core interface on which it is based. NOTE: Additional implementation classes are part of the concurrency utilities (see Chapter 9). The framework’s implementation classes also include the abstract AbstractCollection, AbstractList, AbstractSequentialList, AbstractSet, AbstractQueue, and AbstractMap classes. These classes offer skeletal implementations of the core interfaces to facilitate the creation of concrete implementation classes. Finally, the framework provides two utility classes: Arrays and Collections. Comparable Versus Comparator A collection implementation stores its elements in some order (arrangement). This order may be unsorted, or it may be sorted according to some criterion (such as alphabetical, numerical, or chronological). A sorted collection implementation defaults to storing its elements according to their natural ordering. For example, the natural ordering of String objects is lexicographic or dictionary (also known as alphabetical) order. CHAPTER 8: Discovering the Collections Framework 317 A collection cannot rely on equals() to dictate natural ordering because this method can only determine if two elements are equivalent. Instead, element classes must implement the java.lang.Comparable interface and its int compareTo(T o) method. NOTE: According to Comparable’s JDK documentation, this interface is considered to be part of the collections framework, even though it is a member of the java.lang package. A sorted collection uses compareTo() to determine the natural ordering of this method’s element argument o in a collection. compareTo() compares argument o with the current element (which is the element on which compareTo() was called) and does the following:  It returns a negative value when the current element should precede o.  It returns a zero value when the current element and o are the same.  It returns a positive value when the current element should succeed o. When you need to implement Comparable’s compareTo() method, there are some rules that you must follow. These rules, listed next, are similar to those shown in Chapter 3 for implementing the equals() method:  compareTo() must be reflexive: For any nonnull reference value x, x.compareTo(x) must return 0.  compareTo() must be symmetric: For any nonnull reference values x and y, x.compareTo(y) == -y.compareTo(x) must hold.  compareTo() must be transitive: For any nonnull reference values x, y, and z, if x.compareTo(y) > 0 is true, and if y.compareTo(z) > 0 is true, then x.compareTo(z) > 0 must also be true. Also, compareTo() should throw NullPointerException when the null reference is passed to this method. However, you do not need to check for null because this method throws NullPointerException when it attempts to access a null reference’s members. NOTE: Prior to Java version 5 and its introduction of generics, compareTo()’s argument was of type Object and had to be cast to the appropriate type before the comparison could be made. The cast operator would throw a java.lang.ClassCastException instance when the argument’s type was not compatible with the cast. You might occasionally need to store in a collection objects that are sorted in some order that differs from their natural ordering. In this case, you would supply a comparator to provide that ordering. A comparator is an object whose class implements the Comparator interface. This interface, whose generic type is Comparator, provides the following pair of methods: CHAPTER 8: Discovering the Collections Framework 318  int compare(T o1, T o2) compares both arguments for order. This method returns 0 when o1 equals o2, a negative value when o1 is less than o2, and a positive value when o1 is greater than o2.  boolean equals(Object o) returns true when o “equals” this Comparator in that o is also a Comparator and imposes the same ordering. Otherwise, this method returns false. NOTE: Comparator declares equals() because this interface places an extra condition on this method’s contract. Additionally, this method can return true only if the specified object is also a comparator and it imposes the same ordering as this comparator. You do not have to override equals(), but doing so may improve performance by allowing programs to determine that two distinct comparators impose the same order. Chapter 5 provided an example that illustrated implementing Comparable, and you will discover an additional example later in this chapter. Also, this chapter will present examples of implementing Comparator. Iterable and Collection Most of the core interfaces are rooted in Iterable and its Collection subinterface. Their generic types are Iterable and Collection. Iterable describes any object that can return its contained objects in some sequence. This interface declares an Iterator iterator() method that returns an Iterator instance for iterating over all of the contained objects. Collection represents a collection of objects that are known as elements. This interface provides methods that are common to the Collection subinterfaces on which many collections are based. Table 8–1 describes these methods. Table 8–1. Collection Methods Method Description boolean add(E e) Add element e to this collection. Return true if this collection was modified as a result; otherwise, return false. (Attempting to add e to a collection that does not permit duplicates and already contains a same-valued element results in e not being added.) This method throws java.lang.UnsupportedOperationException when add() is not supported, ClassCastException when e’s class is not appropriate for this collection, java.lang.IllegalArgumentException when some property of e prevents it from being added to this collection, and java.lang.NullPointerException when e contains the null reference and this collection does not support null elements. CHAPTER 8: Discovering the Collections Framework 319 Method Description boolean addAll(Collection c) Add all elements of collection c to this collection. Return true if this collection was modified as the result; otherwise, return false. This method throws UnsupportedOperationException when this collection does not support addAll(), ClassCastException when the class of one of c’s elements is inappropriate for this collection, IllegalArgumentException when some property of an element prevents it from being added to this collection, and NullPointerException when c contains the null reference or when one of its elements is null and this collection does not support null elements. void clear() Remove all elements from this collection. This method throws UnsupportedOperationException when this collection does not support clear(). boolean contains (Object o) Return true when this collection contains o; otherwise, return false. This method throws ClassCastException when the class of o is inappropriate for this collection, and NullPointerException when o contains the null reference and this collection does not support null elements. boolean containsAll (Collection c) Return true when this collection contains all of the elements that are contained in the collection specified by c; otherwise, return false. This method throws ClassCastException when the class of one of c’s elements is inappropriate for this collection, and NullPointerException when c contains the null reference or when one of its elements is null and this collection does not support null elements. boolean equals (Object o) Compare o with this collection and return true when o equals this collection; otherwise, return false. int hashCode() Return this collection’s hash code. Equal collections have equal hash codes. boolean isEmpty() Return true when this collection contains no elements; otherwise, return false. Iterator iterator() Return an Iterator instance for iterating over all of the elements contained in this collection. This Iterable method is redeclared in Collection for convenience. boolean remove (Object o) Remove the element identified as o from this collection. Return true when the element is removed; otherwise, return false. This method throws UnsupportedOperationException when this collection does not support remove(), ClassCastException when the class of o is inappropriate for this collection, and NullPointerException when o contains the null reference and this collection does not support null elements. CHAPTER 8: Discovering the Collections Framework 320 Method Description boolean removeAll (Collection c) Remove all of the elements from this collection that are also contained in collection c. Return true when this collection is modified by this operation; otherwise, return false. This method throws UnsupportedOperationException when this collection does not support removeAll(), ClassCastException when the class of one of c’s elements is inappropriate for this collection, and NullPointerException when c contains the null reference or when one of its elements is null and this collection does not support null elements. boolean retainAll (Collection c) Retain all of the elements in this collection that are also contained in collection c. Return true when this collection is modified by this operation; otherwise, return false. This method throws UnsupportedOperationException when this collection does not support retainAll(), ClassCastException when the class of one of c’s elements is inappropriate for this collection, and NullPointerException when c contains the null reference or when one of its elements is null and this collection does not support null elements. int size() Return the number of elements contained in this collection, or Integer.MAX_VALUE when there are more than Integer.MAX_VALUE elements contained in the collection. Object[] toArray() Return an array containing all of the elements stored in this collection. If this collection makes any guarantees as to what order its elements are returned in by its iterator, this method returns the elements in the same order. The returned array is “safe” in that no references to it are maintained by this collection. (In other words, this method allocates a new array even when this collection is backed by an array.) The caller can safely modify the returned array. T[] toArray(T[] a) Return an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection fits in the specified array, it is returned in the array. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this collection. This method throws NullPointerException when null is passed to a, and java.lang.ArrayStoreException when a’s runtime type is not a supertype of the runtime type of every element in this collection. Table 8–1 reveals three exceptional things about various Collection methods. First, some methods can throw instances of the UnsupportedOperationException class. For example, add() throws UnsupportedOperationException when you attempt to add an object to an immutable (unmodifiable) collection (discussed later in this chapter). CHAPTER 8: Discovering the Collections Framework 321 Second, some of Collection’s methods can throw instances of the ClassCastException class. For example, remove() throws ClassCastException when you attempt to remove an entry (also known as mapping) from a tree-based map whose keys are Strings, but specify a non-String key instead. Finally, Collection’s add() and addAll() methods throw IllegalArgumentException instances when some property (attribute) of the element to be added prevents it from being added to this collection. For example, a third-party collection class’s add() and addAll() methods might throw this exception when they detect negative Integer values. NOTE: Perhaps you are wondering why remove() is declared to accept any Object argument instead of accepting only objects whose types are those of the collection. In other words, why is remove() not declared as boolean remove(E e)? Also, why are containsAll(), removeAll(), and retainAll() not declared with an argument of type Collection, to ensure that the collection argument only contains elements of the same type as the collection on which these methods are called? The answer to these questions is the need to maintain backward compatibility. The collections framework was introduced prior to Java version 5 and its introduction of generics. To let legacy code written before version 5 continue to compile, these four methods were declared with weaker type constraints. Iterator and the Enhanced For Loop Statement By extending Iterable, Collection inherits that interface’s iterator() method, which makes it possible to iterate over a collection. iterator() returns an instance of a class that implements the Iterator interface, whose generic type is expressed as Iterator and which declares the following three methods:  boolean hasNext() returns true when this Iterator instance has more elements to return; otherwise, this method returns false.  E next() returns the next element from the collection associated with this Iterator instance, or throws NoSuchElementException when there are no more elements to return.  void remove() removes the last element returned by next() from the collection associated with this Iterator instance. This method can be called only once per next() call. The behavior of an Iterator instance is unspecified when the underlying collection is modified while iteration is in progress in any way other than by calling remove(). This method throws UnsupportedOperationException when it is not supported by this Iterator, and java.lang.IllegalStateException when remove() has been called without a previous call to next() or when multiple remove() calls occur with no intervening next() calls. CHAPTER 8: Discovering the Collections Framework 322 Listing 8–1 shows you how to iterate over a collection after calling iterator() to return an Iterator instance. The while loop repeatedly calls the iterator’s hasNext() method to determine whether or not iteration should continue, and (if it should continue) the next() method to return the next element from the associated collection. Listing 8–1. Classic collection iteration via the while loop idiom Collection col = ... // This code does not compile because of the ... // Add elements to col. Iterator iter = col.iterator(); while (iter.hasNext()) System.out.println(iter.next()); Because this idiom is commonly used, Java version 5 introduced syntactic sugar to the for loop statement to simplify iteration in terms of the idiom. This sugar makes this statement appear like the foreach statement found in languages such as Perl, and is revealed in Listing 8–2’s simplified equivalent of Listing 8–1. Listing 8–2. Simplified collection iteration via the enhanced for loop statement Collection col = ... // This code does not compile because of the ... // Add elements to col. for (String s: col) System.out.println(s); This sugar hides col.iterator(), a method call that returns an Iterator instance for iterating over col’s elements. It also hides calls to Iterator’s hasNext() and next() methods on this instance. You interpret this sugar to read as follows: “for each String object in col, assign this object to s at the start of the loop iteration.” NOTE: The enhanced for loop statement is also useful in an arrays context, in which it hides the array index variable. Consider the following code fragment: String[] verbs = { "run", "walk", "jump" }; for (String verb: verbs) System.out.println (verb); This code fragment, which reads as “for each String object in the verbs array, assign that object to verb at the start of the loop iteration,” is equivalent to the following code fragment: String[] verbs = { "run", "walk", "jump" }; for (int i = 0; i < verbs.length; i++) System.out.println (verbs[i]); The enhanced for loop statement is limited in that you cannot use this statement where access to the iterator is required to remove an element from a collection. Also, it is not usable where you must replace elements in a collection/array during a traversal, and it cannot be used where you must iterate over multiple collections or arrays in parallel. CHAPTER 8: Discovering the Collections Framework 323 Autoboxing and Unboxing Developers who believe that Java should support only reference types have complained about Java’s support for primitive types. One area where the dichotomy of Java’s type system is clearly seen is the collections framework: you can store objects but not primitive type–based values in collections. Although you cannot directly store a primitive type–based value in a collection, you can indirectly store this value by first wrapping it in an object created from a primitive wrapper class such as Integer, and then storing this primitive wrapper class instance in the collection—see Listing 8–3. (Chapter 6 discusses Java’s primitive wrapper classes.) Listing 8–3. Wrapping an int in an Integer, which is then stored in the collection Collection col = ...; // This code does not compile because of the ... int x = 27; col.add(new Integer(x)); // Indirectly store int value 27 via an Integer object. The reverse situation is also tedious. When you want to retrieve the int from col, you must invoke Integer’s intValue() method (which, if you recall, is inherited from Integer’s Number superclass). Continuing on from Listing 8–3, you would specify int y = col.iterator().next().intValue(); to assign the stored 32-bit integer to y. To alleviate this tedium, Java version 5 introduced autoboxing and unboxing, which are a pair of complementary syntactic sugar–based language features that make primitive values appear more like objects. (This “sleight of hand” is not complete because you cannot specify expressions such as 27.doubleValue().) Autoboxing automatically boxes (wraps) a primitive value in an object of the appropriate primitive wrapper class type whenever a primitive type is specified but a reference is required. For example, you could change Listing 8–3’s third line to col.add(x); and have the compiler box x into an Integer object. Unboxing automatically unboxes (unwraps) a primitive value from its wrapper object whenever a reference is specified but a primitive type is required. For example, you could specify int y = col.iterator().next(); and have the compiler unbox the returned Integer object to int value 27 prior to the assignment. Although autoboxing and unboxing were introduced to simplify working with primitive values in a collections context, these language features can be used in other contexts, and this arbitrary use can lead to a problem that is difficult to understand without knowledge of what is happening behind the scenes. For example, consider Listing 8–4. Listing 8–4. The trouble with autoboxing and unboxing public static void main(String[] args) { Integer i1 = 127; Integer i2 = 127; System.out.println(i1 == i2); // Output: true System.out.println(i1 < i2); // Output: false System.out.println(i1 > i2); // Output: false System.out.println(i1 + i2); // Output: 254 i1 = 30000; CHAPTER 8: Discovering the Collections Framework 324 i2 = 30000; System.out.println(i1 == i2); // Output: false System.out.println(i1 < i2); // Output: false System.out.println(i1 > i2); // Output: false i2 = 30001; System.out.println(i1 < i2); // Output: true System.out.println(i1 + i2); // Output: 60001 } With one exception, this listing’s output is as expected. The exception is the i1 == i2 comparison where each of i1 and i2 contains 30000. Instead of returning true, as is the case where each of i1 and i2 contains 127, i1 == i2 returns false. What is causing this problem? Examine the generated code and you will discover that Integer i1 = 127; is converted to Integer i1 = Integer.valueOf(127); and Integer i2 = 127; is converted to Integer i2 = Integer.valueOf(127);. According to valueOf()’s Java documentation, this method takes advantage of caching to improve performance. NOTE: valueOf() is also used when adding a primitive value to a collection. For example, col.add(27) is converted to col.add(Integer.valueOf(27)). Integer maintains an internal cache of unique Integer objects over a small range of values. The low bound of this range is -128, and the high bound defaults to 127. However, you can change the high bound by assigning a different value to system property java.lang.Integer.IntegerCache.high (via the System class’s String setProperty(String prop, String value) method—I demonstrated this method’s getProperty() counterpart in Chapter 7). NOTE: Each of Byte, Long, and Short also maintains an internal cache of unique Byte, Long, and Short objects, respectively. Because of the cache, each Integer.valueOf(127) call returns the same Integer object reference, which is why i1 == i2 (which compares references) evaluates to true. Because 30000 lies outside of the default range, each Integer.valueOf(30000) call returns a reference to a new Integer object, which is why i1 == i2 evaluates to false. In contrast to == and !=, which do not unbox the boxed values prior to the comparison, operators such as <, >, and + unbox these values before performing their operations. As a result, i1 < i2 is converted to i1.intValue() < i2.intValue() and i1 + i2 is converted to i1.intValue() + i2.intValue(). CAUTION: Do not assume that autoboxing and unboxing are used in the context of the == and != operators. This book was purchased by harshamv@variable3.comCHAPTER 8: Discovering the Collections Framework 325 List A list is an ordered collection, which is also known as a sequence. Elements can be stored in and accessed from specific locations via integer indexes. Some of these elements may be duplicates or null (when the list’s implementation allows null elements). Lists are described by the List interface, whose generic type is List. List extends Collection and redeclares its inherited methods, partly for convenience. It also redeclares iterator(), add(), remove(), equals(), and hashCode() to place extra conditions on their contracts. For example, List’s contract for add() specifies that it appends an element to the end of the list, rather than adding the element to the collection. List also declares Table 8–2’s list-specific methods. Table 8–2. List-specific Methods Method Description void add(int index, E e) Insert element e into this list at position index. Shift the element currently at this position (if any) and any subsequent elements to the right. This method throws UnsupportedOperationException when this list does not support add(), ClassCastException when e’s class is inappropriate for this list, IllegalArgumentException when some property of e prevents it from being added to this list, NullPointerException when e contains the null reference and this list does not support null elements, and java.lang.IndexOutOfBoundsException when index is less than 0 or index is greater than size(). boolean addAll (int index, Collection c) Insert all of c’s elements into this list starting at position index. Shift the element currently at this position (if any) and any subsequent elements to the right. The new elements appear in this list in the order in which they are returned by c’s iterator. This method throws UnsupportedOperationException when this list does not support addAll(), ClassCastException when the class of one of c’s elements is inappropriate for this list, IllegalArgumentException when some property of an element prevents it from being added to this list, NullPointerException when c contains the null reference or when one of its elements is null and this list does not support null elements, and IndexOutOfBoundsException when index is less than 0 or index is greater than size(). E get(int index) Return the element stored in this list at position index. This method throws IndexOutOfBoundsException when index is less than 0 or index is greater than or equal to size(). int indexOf (Object o) Return the index of the first occurrence of element o in this list, or -1 when this list does not contain the element. This method throws ClassCastException when o’s class is inappropriate for this list, and NullPointerException when o contains the null reference and this list does not support null elements. CHAPTER 8: Discovering the Collections Framework 326 Method Description int lastIndexOf (Object o) Return the index of the last occurrence of element o in this list, or -1 when this list does not contain the element. This method throws ClassCastException when o’s class is inappropriate for this list, and NullPointerException when o contains the null reference and this list does not support null elements. ListIterator listIterator() Return a list iterator over the elements in this list. The elements are returned in the same order as they appear in the list. ListIterator listIterator(int index) Return a list iterator over the elements in this list starting with the element located at index. The elements are returned in the same order as they appear in the list. This method throws IndexOutOfBoundsException when index is less than 0 or index is greater than size(). E remove(int index) Remove the element at position index from this list and return this element. This method throws UnsupportedOperationException when this list does not support remove(), and IndexOutOfBoundsException when index is less than 0 or index is greater than or equal to size(). E set(int index, E e) Replace the element at position index in this list with element e and return the element previously stored at this position. This method throws UnsupportedOperationException when this list does not support set(), ClassCastException when e’s class is inappropriate for this list, IllegalArgumentException when some property of e prevents it from being added to this list, NullPointerException when e contains the null reference and this list does not support null elements, and IndexOutOfBoundsException when index is less than 0 or index is greater than or equal to size(). List subList (int fromIndex, int toIndex) Return a view (discussed later) of the portion of this list between fromIndex (inclusive) and toIndex (exclusive). (If fromIndex and toIndex are equal, the returned list is empty.) The returned list is backed by this list, so nonstructural changes in the returned list are reflected in this list and vice versa. The returned list supports all of the optional list methods (those methods that can throw UnsupportedOperationException) supported by this list. This method throws IndexOutOfBoundsException when fromIndex is less than 0, toIndex is greater than size(), or fromIndex is greater than toIndex. Table 8–2 refers to the ListIterator interface, which is more flexible than its Iterator superinterface in that ListIterator provides methods for iterating over a list in either direction, modifying the list during iteration, and obtaining the iterator’s current position in the list. CHAPTER 8: Discovering the Collections Framework 327 NOTE: The Iterator and ListIterator instances that are returned by the iterator() and listIterator() methods in the ArrayList and LinkedList List implementation classes are fail-fast: when a list is structurally modified (by calling the implementation’s add() method to add a new element, for example) after the iterator is created, in any way except through the iterator's own add() or remove() methods, the iterator throws java.lang.ConcurrentModificationException. Therefore, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, nondeterministic behavior at an undetermined time in the future. ListIterator declares the following methods:  void add(E e) inserts e into the list being iterated over. This element is inserted immediately before the next element that would be returned by next(), if any, and after the next element that would be returned by previous(), if any. This method throws UnsupportedOperationException when this list does not support add(), ClassCastException when e’s class is inappropriate for this list, and IllegalArgumentException when some property of e prevents it from being added to this list.  boolean hasNext() returns true when this list iterator has more elements when traversing the list in the forward direction.  boolean hasPrevious() returns true when this list iterator has more elements when traversing the list in the reverse direction.  E next() returns the next element in this list. This method throws NoSuchElementException when there is no next element.  int nextIndex() returns the index of the element that would be returned by a subsequent call to next(), or the size of the list when at the end of the list.  E previous() returns the previous element in this list. This method throws NoSuchElementException when there is no previous element.  int previousIndex() returns the index of the element that would be returned by a subsequent call to previous(), or -1 when at the beginning of the list.  void remove() removes from this list the last element that was returned by next() or previous(). This call can be made only once per call to next() or previous(). Furthermore, it can be made only when add() has not been called after the last call to next() or previous(). This method throws UnsupportedOperationException when this list does not support remove(), and IllegalStateException when neither next() nor previous() has been called, or remove() or add() has already been called after the last call to next() or previous(). CHAPTER 8: Discovering the Collections Framework 328  void set(E e) replaces the last element returned by next() or previous() with element e. This call can be made only when neither remove() nor add() has been called after the last call to next() or previous(). This method throws UnsupportedOperationException when this list does not support set(), ClassCastException when e’s class is inappropriate for this list, IllegalArgumentException when some property of e prevents it from being added to this list, and IllegalStateException when neither next() nor previous() has been called, or remove() or add() has already been called after the last call to next() or previous(). A ListIterator instance does not have the concept of a current element. Instead, it has the concept of a cursor for navigating through a list. The nextIndex() and previousIndex() methods return the cursor position, which always lies between the element that would be returned by a call to previous() and the element that would be returned by a call to next(). A list iterator for a list of length n has n+1 possible cursor positions, as illustrated by each caret (^) below: Element(0) Element(1) Element(2) ... Element(n-1) cursor positions: ^ ^ ^ ^ ^ NOTE: You can mix calls to next() and previous() as long as you are careful. Keep in mind that the first call to previous() returns the same element as the last call to next(). Furthermore, the first call to next() following a sequence of calls to previous() returns the same element as the last call to previous(). Table 8–2’s description of the subList() method refers to the concept of a view, which is a list that is backed by another list. Changes that are made to the view are reflected in this backing list. The view can cover the entire list or, as subList()’s name implies, only part of the list. The subList() method is useful for performing range-view operations over a list in a compact manner. For example, list.subList(fromIndex, toIndex).clear(); removes a range of elements from list where the first element is located at fromIndex and the last element is located at toIndex-1. CAUTION: A view’s meaning becomes undefined when changes are made to the backing list. Therefore, you should only use subList() temporarily, whenever you need to perform a sequence of range operations on the backing list. CHAPTER 8: Discovering the Collections Framework 329 ArrayList The ArrayList class provides a list implementation that is based on an internal array. As a result, access to the list’s elements is fast. However, because elements must be moved to open a space for insertion or to close a space after deletion, insertions and deletions of elements is slow. NOTE: Refer to Chapter 2 for an introduction to arrays. ArrayList supplies three constructors:  ArrayList() creates an empty array list with an initial capacity (storage space) of ten elements. Once this capacity is reached, a larger array is allocated, elements from the current array are copied into the larger array, and the larger array becomes the new current array. This process repeats as additional elements are added to the array list.  ArrayList(Collection collection) creates an array list containing collection’s elements in the order in which they are returned by the collection’s iterator.  ArrayList(int capacity) creates an empty array list with an initial capacity of capacity elements. Listing 8–5 demonstrates an array list. Listing 8–5. A demonstration of an array-based list import java.util.ArrayList; import java.util.List; public class ArrayListDemo { public static void main(String[] args) { List ls = new ArrayList(); String[] weekDays = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"}; for (String weekDay: weekDays) ls.add(weekDay); dump("ls:", ls); ls.set(ls.indexOf("Wed"), "Wednesday"); dump("ls:", ls); ls.remove(ls.lastIndexOf("Fri")); dump("ls:", ls); } static void dump(String title, List ls) { System.out.print(title + " "); for (String s: ls) System.out.print(s + " "); System.out.println(); } } CHAPTER 8: Discovering the Collections Framework 330 The dump() method’s enhanced for loop statement uses iterator(), hasNext(), and next() behind the scenes. When you run this application, it generates the following output: ls: Sun Mon Tue Wed Thu Fri Sat ls: Sun Mon Tue Wednesday Thu Fri Sat ls: Sun Mon Tue Wednesday Thu Sat LinkedList The LinkedList class provides a list implementation that is based on linked nodes. Because links must be traversed, access to the list’s elements is slow. However, because only node references need to be changed, insertions and deletions of elements are fast. WHAT IS A NODE? A node is a fixed sequence of value and link memory locations. Unlike an array, where each slot stores a single value of the same primitive type or reference supertype, a node can store multiple values of different types. It can also store links (references to other nodes). Consider the following simple Node class: class Node { // value field String name; // link field Node next; } Node describes simple nodes where each node consists of a single name value field and a single next link field. Notice that next is of the same type as the class in which it is declared. This arrangement lets a node instance store a reference to another node instance (which is the next node) in this field. The resulting nodes are linked together. The following code fragment creates a couple of Node objects and links the second Node object to the first Node object. This fragment also demonstrates how to traverse this linked list by following each Node object’s next field. Node traversal stops when the traversal code discovers that next contains the null reference, which signifies the end of the list. Node first = new Node(); first.name = "First node"; Node last = new Node(); last.name = "Last node"; last.next = null; first.next = last; Node temp = first; while (temp != null) { System.out.println(temp.name); temp = temp.next; } CHAPTER 8: Discovering the Collections Framework 331 The code first builds a linked list of two Nodes, and then assigns first to local variable temp in order to traverse the list without losing the reference to the first node that is stored in first. While temp is not null, the loop outputs the name field. It also navigates to the next Node object in the list via the temp = temp.next; statement. If you convert this code into an application and run the application, you will discover the following output: First node Last node LinkedList supplies two constructors:  LinkedList() creates an empty linked list.  LinkedList(Collection collection) creates a linked list containing collection’s elements in the order in which they are returned by the collection’s iterator. Listing 8–6 demonstrates a linked list. Listing 8–6. A demonstration of a list of linked nodes import java.util.LinkedList; import java.util.List; import java.util.ListIterator; public class LinkedListDemo { public static void main(String[] args) { List ls = new LinkedList(); String[] weekDays = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"}; for (String weekDay: weekDays) ls.add(weekDay); dump("ls:", ls); ls.add(1, "Sunday"); ls.add(3, "Monday"); ls.add(5, "Tuesday"); ls.add(7, "Wednesday"); ls.add(9, "Thursday"); ls.add(11, "Friday"); ls.add(13, "Saturday"); dump("ls:", ls); ListIterator li = ls.listIterator(ls.size()); while (li.hasPrevious()) System.out.print(li.previous() + " "); System.out.println(); } static void dump(String title, List ls) { System.out.print(title + " "); for (String s: ls) System.out.print(s + " "); System.out.println(); } } CHAPTER 8: Discovering the Collections Framework 332 This application demonstrates that each successive add() method call must increase its index by 2 to account for the previously added element when adding longer weekday names to the list. It also shows you how to output a list in reverse order: return a list iterator with its cursor initialized past the end of the list and repeatedly call previous(). When you run this application, it generates the following output: ls: Sun Mon Tue Wed Thu Fri Sat ls: Sun Sunday Mon Monday Tue Tuesday Wed Wednesday Thu Thursday Fri Friday Sat Saturday Saturday Sat Friday Fri Thursday Thu Wednesday Wed Tuesday Tue Monday Mon Sunday Sun Set A set is a collection that contains no duplicate elements. In other words, a set contains no pair of elements e1 and e2 such that e1.equals(e2) returns true. Furthermore, a set can contain at most one null element. Sets are described by the Set interface, whose generic type is Set. Set extends Collection and redeclares its inherited methods, for convenience and also to add stipulations to the contracts for add(), equals(), and hashCode(), to address how they behave in a set context. Also, Set’s documentation states that all constructors of implementation classes must create sets that contain no duplicate elements. Set does not introduce new methods. TreeSet The TreeSet class provides a set implementation that is based on a tree data structure. As a result, elements are stored in sorted order. However, accessing these elements is somewhat slower than with the other Set implementations (which are not sorted) because links must be traversed. NOTE: Check out Wikipedia’s “Tree (data structure)” entry (http://en.wikipedia.org/wiki/Tree_%28data_structure%29) to learn about trees. TreeSet supplies four constructors:  TreeSet() creates a new, empty tree set that is sorted according to the natural ordering of its elements. All elements inserted into the set must implement the Comparable interface.  TreeSet(Collection collection) creates a new tree set containing collection’s elements, sorted according to the natural ordering of its elements. All elements inserted into the new set must implement the Comparable interface. This constructor throws ClassCastException when collection’s elements do not implement Comparable or are not mutually comparable, and NullPointerException when collection contains the null reference. CHAPTER 8: Discovering the Collections Framework 333  TreeSet(Comparator comparator) creates a new, empty tree set that is sorted according to the specified comparator. Passing null to comparator implies that natural ordering will be used.  TreeSet(SortedSet sortedSet) creates a new tree set containing the same elements and using the same ordering as sortedSet. (I discuss sorted sets later in this chapter.) This constructor throws NullPointerException when sortedSet contains the null reference. Listing 8–7 demonstrates a tree set. Listing 8–7. A demonstration of a tree set with String elements sorted according to their natural ordering import java.util.Set; import java.util.TreeSet; public class TreeSetDemo { public static void main(String[] args) { Set ss = new TreeSet(); String[] fruits = {"apples", "pears", "grapes", "bananas", "kiwis"}; for (String fruit: fruits) ss.add(fruit); dump("ss:", ss); } static void dump(String title, Set ss) { System.out.print(title + " "); for (String s: ss) System.out.print(s + " "); System.out.println(); } } Because String implements Comparable, it is legal for this application to use the TreeSet() constructor to insert the contents of the fruits array into the set. When you run this application, it generates the following output: ss: apples bananas grapes kiwis pears HashSet The HashSet class provides a set implementation that is backed by a hashtable data structure (implemented as a HashMap instance, discussed later, which provides a quick way to determine if an element has already been stored in this structure). Although this class provides no ordering guarantees for its elements, HashSet is much faster than TreeSet. Furthermore, HashSet permits the null reference to be stored in its instances. NOTE: Check out Wikipedia’s “Hash table” entry (http://en.wikipedia.org/wiki/Hash_table) to learn about hashtables. CHAPTER 8: Discovering the Collections Framework 334 HashSet supplies four constructors:  HashSet() creates a new, empty hashset where the backing HashMap instance has an initial capacity of 16 and a load factor of 0.75. You will learn what these items mean when I discuss HashMap later in this chapter.  HashSet(Collection collection) creates a new hashset containing collection’s elements. The backing HashMap has an initial capacity sufficient to contain collection’s elements and a load factor of 0.75. This constructor throws NullPointerException when collection contains the null reference.  HashSet(int initialCapacity) creates a new, empty hashset where the backing HashMap instance has the capacity specified by initialCapacity and a load factor of 0.75. This constructor throws IllegalArgumentException when initialCapacity’s value is less than 0.  HashSet(int initialCapacity, float loadFactor) creates a new, empty hashset where the backing HashMap instance has the capacity specified by initialCapacity and the load factor specified by loadFactor. This constructor throws IllegalArgumentException when initialCapacity is less than 0 or when loadFactor is less than or equal to 0. Listing 8–8 demonstrates a hashset. Listing 8–8. A demonstration of a hashset with String elements unordered import java.util.HashSet; import java.util.Set; public class HashSetDemo { public static void main(String[] args) { Set ss = new HashSet(); String[] fruits = {"apples", "pears", "grapes", "bananas", "kiwis", "pears", null}; for (String fruit: fruits) ss.add(fruit); dump("ss:", ss); } static void dump(String title, Set ss) { System.out.print(title + " "); for (String s: ss) System.out.print(s + " "); System.out.println(); } } In Listing 8–7’s TreeSetDemo application, I did not add null to the fruits array because TreeSet throws NullPointerException when it detects an attempt to add this element. In This book was purchased by harshamv@variable3.comCHAPTER 8: Discovering the Collections Framework 335 contrast, HashSet permits null to be added, which is why Listing 8–8 includes null in HashSetDemo’s fruits array. When you run this application, it generates unordered output such as the following: ss: null grapes bananas kiwis pears apples Suppose you want to add instances of your classes to a hashset. As with String, your classes must override equals() and hashCode(); otherwise, duplicate class instances can be stored in the hashset. For example, Listing 8–9 presents the source code to an application whose Planet class overrides equals() but fails to also override hashCode(). Listing 8–9. A custom Planet class not overriding hashCode() import java.util.HashSet; import java.util.Set; public class CustomClassAndHashSet { public static void main(String[] args) { Set sp = new HashSet(); sp.add(new Planet("Mercury")); sp.add(new Planet("Venus")); sp.add(new Planet("Earth")); sp.add(new Planet("Mars")); sp.add(new Planet("Jupiter")); sp.add(new Planet("Saturn")); sp.add(new Planet("Uranus")); sp.add(new Planet("Neptune")); sp.add(new Planet("Fomalhaut b")); Planet p1 = new Planet("51 Pegasi b"); sp.add(p1); Planet p2 = new Planet("51 Pegasi b"); sp.add(p2); System.out.println(p1.equals(p2)); System.out.println(sp); } } class Planet { private String name; Planet(String name) { this.name = name; } @Override public boolean equals(Object o) { if (!(o instanceof Planet)) return false; Planet p = (Planet) o; return p.name.equals(name); } String getName() { return name; CHAPTER 8: Discovering the Collections Framework 336 } @Override public String toString() { return name; } } Listing 8–9’s Planet class declares a single name field of type String. Although it might seem pointless to declare Planet with a single String field because I could refactor this listing to remove Planet and work with String, I might want to introduce additional fields to Planet (perhaps to store a planet’s mass and other characteristics) in the future. NOTE: equals() relies on a little known fact about the Java language: one instance’s private members can be accessed from another instance of the same class. For example, equals() can specify p.name to access p’s private name field. Directly accessing an instance’s private members in this manner is legal because encapsulation is not violated. When you run this application, it generates unordered output such as the following: true [Jupiter, Fomalhaut b, Neptune, Uranus, Venus, Earth, Mercury, 51 Pegasi b, Mars, Saturn, 51 Pegasi b] This output reveals two 51 Pegasi b elements in the hashset. Although these elements are equal from the perspective of the overriding equals() method (the first output line, true, proves this point), overriding equals() is not enough to avoid duplicate elements being stored in a hashset: you must also override hashCode(). The easiest way to override hashCode() in Listing 8–9’s Planet class is to have the overriding method call the name field’s hashCode() method and return its value. (This technique only works with a class whose single reference field’s class provides a valid hashCode() method.) Listing 8–10 presents this overriding hashCode() method. Listing 8–10. Handing over hash code calculation to another hashCode() method @Override public int hashCode() { return name.hashCode(); } Introduce this method into the previous Planet class and run the application. You will observe output (similar to that shown below) that reveals no duplicate elements: true [Saturn, Earth, Uranus, Fomalhaut b, 51 Pegasi b, Venus, Jupiter, Mercury, Mars, Neptune] CHAPTER 8: Discovering the Collections Framework 337 NOTE: LinkedHashSet is a subclass of HashSet that uses a linked list to store its elements. As a result, LinkedHashSet’s iterator returns elements in the order in which they were inserted. For example, if Listing 8–8 had specified Set ss = new LinkedHashSet();, the application’s output would have been ss: apples pears grapes bananas kiwis null. Also, LinkedHashSet offers slower performance than HashSet and faster performance than TreeSet. EnumSet Chapter 5 introduced you to traditional enumerated types and their enum replacement. (An enum is an enumerated type that is expressed via reserved word enum.) Listing 8–11 presents an example of a traditional enumerated type. Listing 8–11. An enumerated type of weekday constants public static final int SUNDAY = 1; public static final int MONDAY = 2; public static final int TUESDAY = 4; public static final int WEDNESDAY = 8; public static final int THURSDAY = 16; public static final int FRIDAY = 32; public static final int SATURDAY = 64; Although the enum has many advantages over the traditional enumerated type, the traditional enumerated type is less awkward to use when combining constants into a set; for example, public static final int DAYS_OFF = SUNDAY | MONDAY;. DAYS_OFF is an example of an integer-based, fixed-length bitset, which is a set of bits where each bit indicates that its associated member belongs to the set when the bit is set to 1, and is absent from the set when the bit is set to 0. NOTE: An int-based bitset cannot contain more than 32 members because int has a size of 32 bits. Similarly, a long-based bitset cannot contain more than 64 members because long has a size of 64 bits. This bitset is formed by bitwise inclusive ORing the traditional enumerated type’s integer constants together via the bitwise inclusive OR operator (|): you could also use +. Each constant must be a unique power of two (starting with one) because otherwise it is impossible to distinguish between the members of this bitset. To determine if a constant belongs to the bitset, create an expression that involves the bitwise AND operator (&). For example, ((DAYS_OFF & MONDAY) == MONDAY) bitwise ANDs DAYS_OFF (3) with MONDAY (2), which results in 2. This value is compared via == with MONDAY (2), and the result of the expression is true: MONDAY is a member of the DAYS_OFF bitset. CHAPTER 8: Discovering the Collections Framework 338 You can accomplish the same task with an enum by instantiating an appropriate Set implementation class and calling the add() method multiple times to store the constants in the set. Listing 8–12 illustrates this more awkward alternative. Listing 8–12. Creating the Set equivalent of DAYS_OFF import java.util.Set; import java.util.TreeSet; enum Weekday { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY } class DaysOff { public static void main(String[] args) { Set daysOff = new TreeSet(); daysOff.add(Weekday.SUNDAY); daysOff.add(Weekday.MONDAY); System.out.println(daysOff); } } When you run this application, it generates the following output: [SUNDAY, MONDAY] NOTE: The constants’ ordinals and not their names are stored in the tree set, which is why the names appear unordered even though the constants are stored in sorted order of their ordinals. In addition to being more awkward to use (and verbose) than the bitset, the Set alternative requires more memory to store each constant and is not as fast. Because of these problems, EnumSet was introduced. The EnumSet class provides a Set implementation that is based on a bitset. Its elements are constants that must come from the same enum, which is specified when the enum set is created. Null elements are not permitted; any attempt to store a null element results in a thrown NullPointerException. Listing 8–13 demonstrates EnumSet. Listing 8–13. Creating the EnumSet equivalent of DAYS_OFF import java.util.EnumSet; import java.util.Iterator; import java.util.Set; enum Weekday { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY } public class EnumSetDemo { public static void main(String[] args) CHAPTER 8: Discovering the Collections Framework 339 { Set daysOff = EnumSet.of(Weekday.SUNDAY, Weekday.MONDAY); Iterator iter = daysOff.iterator(); while (iter.hasNext()) System.out.println(iter.next()); } } EnumSet, whose generic type is EnumSet>, provides a variety of utility methods for conveniently constructing enum sets. For example, > EnumSet of(E e1, E e2) returns an EnumSet instance consisting of elements e1 and e2. In this example, those elements are Weekday.SUNDAY and Weekday.MONDAY. When you run this application, it generates the following output: SUNDAY MONDAY NOTE: In addition to providing several overloaded of() methods, EnumSet provides other methods for conveniently creating enum sets. For example, allOf() returns an EnumSet instance that contains all of an enum’s constants, where this method’s solitary argument is a class literal that identifies the enum: Set allWeekDays = EnumSet.allOf(Weekday.class); Similarly, range() returns an EnumSet instance containing a range of an enum’s elements (with the range’s limits as specified by this method’s two arguments): for (WeekDay wd : EnumSet.range(WeekDay.MONDAY, WeekDay.FRIDAY)) System.out.println(wd); SortedSet TreeSet is an example of a sorted set, which is a set that maintains its elements in ascending order, sorted according to their natural ordering or according to a comparator that is supplied when the sorted set is created. Sorted sets are described by the SortedSet interface. SortedSet, whose generic type is SortedSet, extends Set. With two exceptions, the methods it inherits from Set behave identically on sorted sets as on other sets:  The Iterator instance returned from iterator() traverses the sorted set in order.  The array returned by toArray() contains the sorted set’s elements in order. CHAPTER 8: Discovering the Collections Framework 340 NOTE: Although not guaranteed, the toString() methods of SortedSet implementations in the collections framework (such as TreeSet) return a string containing all of the sorted set’s elements in order. SortedSet’s documentation requires that an implementation must provide the four standard constructors that I presented in my discussion of TreeSet. Furthermore, implementations of this interface must implement the methods that are described in Table 8–3. Table 8–3. SortedSet-specific Methods Method Description Comparator comparator() Return the comparator used to order the elements in this set, or null when this set uses the natural ordering of its elements. E first() Return the first (lowest) element currently in this set, or throw a NoSuchElementException instance when this set is empty. SortedSet headSet(E toElement) Return a view of that portion of this set whose elements are strictly less than toElement. The returned set is backed by this set, so changes in the returned set are reflected in this set and vice versa. The returned set supports all optional set operations that this set supports. This method throws ClassCastException when toElement is not compatible with this set’s comparator (or, when the set has no comparator, when toElement does not implement Comparable), NullPointerException when toElement is null and this set does not permit null elements, and IllegalArgumentException when this set has a restricted range and toElement lies outside of this range’s bounds. E last() Return the last (highest) element currently in this set, or throw a NoSuchElementException instance when this set is empty. SortedSet subSet(E fromElement, E toElement) Return a view of the portion of this set whose elements range from fromElement, inclusive, to toElement, exclusive. (When fromElement and toElement are equal, the returned set is empty.) The returned set is backed by this set, so changes in the returned set are reflected in this set and vice versa. The returned set supports all optional set operations that this set supports. This method throws ClassCastException when fromElement and toElement cannot be compared to one another using this set’s comparator (or, when the set has no comparator, using natural ordering), NullPointerException when fromElement or toElement is null and this set does not permit null elements, and IllegalArgumentException when fromElement is greater than toElement or when this set has a restricted range and fromElement or toElement lies outside of this range’s bounds. CHAPTER 8: Discovering the Collections Framework 341 Method Description SortedSet tailSet(E fromElement) Return a view of that portion of this set whose elements are greater than or equal to fromElement. The returned set is backed by this set, so changes in the returned set are reflected in this set and vice versa. The returned set supports all optional set operations that this set supports. This method throws ClassCastException when fromElement is not compatible with this set’s comparator (or, when the set has no comparator, when fromElement does not implement Comparable), NullPointerException when fromElement is null and this set does not permit null elements, and IllegalArgumentException when this set has a restricted range and fromElement lies outside of the range’s bounds. The set-based range views returned from headSet(), subSet(), and tailSet() are analogous to the list-based range view returned from List’s subList() method except that a set-based range view remains valid even when the backing sorted set is modified. As a result, a set-based range view can be used for a lengthy period of time. NOTE: Unlike a list-based range view whose endpoints are elements in the backing list, the endpoints of a set-based range view are absolute points in element space, allowing a set-based range view to serve as a window onto a portion of the set’s element space. Any changes made to the set-based range view are written back to the backing sorted set and vice versa. Each range view returned by headSet(), subSet(), or tailSet() is half open because it does not include its high endpoint (headSet() and subSet()) or its low endpoint (tailSet()). For the first two methods, the high endpoint is specified by argument toElement; for the last method, the low endpoint is specified by argument fromElement. NOTE: You could also regard the returned range view as being half closed because it includes only one of its endpoints. Listing 8–14 demonstrates a sorted set based on a tree set. Listing 8–14. A sorted set of fruit and vegetable names import java.util.SortedSet; import java.util.TreeSet; public class SortedSetDemo { public static void main(String[] args) { SortedSet sss = new TreeSet(); String[] fruitAndVeg = { "apple", "potato", "turnip", "banana", "corn", "carrot", "cherry", "pear", "mango", "strawberry", "cucumber", "grape", "banana", "kiwi", "radish", "blueberry", "tomato", "onion", "raspberry", CHAPTER 8: Discovering the Collections Framework 342 "lemon", "pepper", "squash", "melon", "zucchini", "peach", "plum", "turnip", "onion", "nectarine" }; System.out.println("Array size = " + fruitAndVeg.length); for (String fruitVeg: fruitAndVeg) sss.add(fruitVeg); dump("sss:", sss); System.out.println("Sorted set size = " + sss.size()); System.out.println("First element = " + sss.first()); System.out.println("Last element = " + sss.last()); System.out.println("Comparator = " + sss.comparator()); dump("hs:", sss.headSet("n")); dump("ts:", sss.tailSet("n")); System.out.println("Count of p-named fruits & vegetables = " + sss.subSet("p", "q").size()); System.out.println("Incorrect count of c-named fruits & vegetables = " + sss.subSet("carrot", "cucumber").size()); System.out.println("Correct count of c-named fruits & vegetables = " + sss.subSet("carrot", "cucumber\0").size()); } static void dump(String title, SortedSet sss) { System.out.print(title + " "); for (String s: sss) System.out.print(s + " "); System.out.println(); } } When you run this application, it generates the following output: Array size = 29 ss: apple banana blueberry carrot cherry corn cucumber grape kiwi lemon mango melon nectarine onion peach pear pepper plum potato radish raspberry squash strawberry tomato turnip zucchini Sorted set size = 26 First element = apple Last element = zucchini Comparator = null hs: apple banana blueberry carrot cherry corn cucumber grape kiwi lemon mango melon ts: nectarine onion peach pear pepper plum potato radish raspberry squash strawberry tomato turnip zucchini Count of p-named fruits & vegetables = 5 Incorrect count of c-named fruits & vegetables = 3 Correct count of c-named fruits & vegetables = 4 This output reveals that the sorted set’s size is less than the array’s size because a set cannot contain duplicate elements: the duplicate banana, turnip, and onion elements are not stored in the sorted set. The comparator() method returns null because the sorted set was not created with a comparator. Instead, the sorted set relies on the natural ordering of String elements to store them in sorted order. CHAPTER 8: Discovering the Collections Framework 343 The headSet() and tailSet() methods are called with argument "n" to return, respectively, a set of elements whose names begin with a letter that is strictly less than n, and a letter that is greater than or equal to n. Finally, the output shows you that you must be careful when passing an upper limit to subSet(). As you can see, ss.subSet("carrot", "cucumber") does not include cucumber in the returned range view because cucumber is subSet()’s high endpoint. To include cucumber in the range view, you need to form a closed range or closed interval (both endpoints are included). With String objects, you accomplish this task by appending \0 to the string. For example, ss.subSet("carrot", "cucumber\0") includes cucumber because it is less than cucumber\0. This same technique can be applied wherever you need to form an open range or open interval (neither endpoint is included). For example, ss.subSet("carrot\0", "cucumber") does not include carrot because it is less than carrot\0. Furthermore, it does not include high endpoint cucumber. NOTE: When you want to create closed and open ranges for elements created from your own classes, you need to provide some form of predecessor() and successor() methods that return an element’s predecessor and successor. You need to be careful when designing classes that work with sorted sets. For example, the class must implement Comparable when you plan to store the class’s instances in a sorted set where these elements are sorted according to their natural ordering. Consider Listing 8–15. Listing 8–15. A custom Employee class not implementing Comparable import java.util.SortedSet; import java.util.TreeSet; public class CustomClassAndSortedSet { public static void main(String[] args) { SortedSet sse = new TreeSet(); sse.add(new Employee("Sally Doe")); sse.add(new Employee("Bob Doe")); // ClassCastException thrown here sse.add(new Employee("John Doe")); System.out.println(sse); } } class Employee { private String name; Employee(String name) { this.name = name; } @Override public String toString() CHAPTER 8: Discovering the Collections Framework 344 { return name; } } When you run this application, it generates the following output: Exception in thread "main" java.lang.ClassCastException: Employee cannot be cast to java.lang.Comparable at java.util.TreeMap.put(Unknown Source) at java.util.TreeSet.add(Unknown Source) at CustomClassAndSortedSet.main(CustomClassAndSortedSet.java:10) The ClassCastException instance is thrown during the second add() method call because the sorted set implementation, an instance of TreeSet, is unable to call the second Employee element’s compareTo() method, because Employee does not implement Comparable. The solution to this problem is to have the class implement Comparable, which is exactly what is revealed in Listing 8–16. Listing 8–16. Making Employee elements comparable import java.util.SortedSet; import java.util.TreeSet; public class CustomClassAndSortedSet { public static void main(String[] args) { SortedSet sse = new TreeSet(); sse.add(new Employee("Sally Doe")); sse.add(new Employee("Bob Doe")); Employee e1 = new Employee("John Doe"); Employee e2 = new Employee("John Doe"); sse.add(e1); sse.add(e2); System.out.println(sse); System.out.println(e1.equals(e2)); } } class Employee implements Comparable { private String name; Employee(String name) { this.name = name; } @Override public int compareTo(Employee e) { return name.compareTo(e.name); } @Override public String toString() { return name; } } This book was purchased by harshamv@variable3.comCHAPTER 8: Discovering the Collections Framework 345 Listing 8–16’s main() method differs from Listing 8–15 in that it also creates two Employee objects initialized to "John Doe", adds these objects to the sorted set, and compares these objects for equality via equals(). Furthermore, Listing 8–16 declares Employee to implement Comparable, introducing a compareTo() method into Employee. When you run this application, it generates the following output: [Bob Doe, John Doe, Sally Doe] false This output shows that only one "John Doe" Employee object is stored in the sorted set. After all, a set cannot contain duplicate elements. However, the false value (resulting from the equals() comparison) also shows that the sorted set’s natural ordering is inconsistent with equals(), which violates SortedSet’s contract: The ordering maintained by a sorted set (whether or not an explicit comparator is provided) must be consistent with equals() if the sorted set is to correctly implement the Set interface. This is so because the Set interface is defined in terms of the equals() operation, but a sorted set performs all element comparisons using its compareTo() (or compare()) method, so two elements that are deemed equal by this method are, from the standpoint of the sorted set, equal. Because the application works correctly, why should SortedSet’s contract matter? Although the contract does not appear to matter with respect to the TreeSet implementation of SortedSet, perhaps it will matter in the context of a third-party class that implements this interface. Listing 8–17 shows you how to correct this problem and make Employee instances work with any implementation of a sorted set. Listing 8–17. A contract-compliant Employee class class Employee implements Comparable { private String name; Employee(String name) { this.name = name; } @Override public int compareTo(Employee e) { return name.compareTo(e.name); } @Override public boolean equals(Object o) { if (!(o instanceof Employee)) return false; Employee e = (Employee) o; return e.name.equals(name); } @Override public String toString() { CHAPTER 8: Discovering the Collections Framework 346 return name; } } Listing 8–17 corrects the SortedSet contract violation by overriding equals(). If you replace Listing 8–16’s Employee class with Listing 8–17, and run the resulting application, you will observe [Bob Doe, John Doe, Sally Doe] as the first line of output and true as the second line: the sorted set’s natural ordering is now consistent with equals(). NOTE: Although it is important to override hashCode() whenever you override equals(), I did not override hashCode() (although I overrode equals()) in Listing 8–17’s Employee class to emphasize that tree-based sorted sets ignore hashCode(). Queue A queue is a collection in which elements are stored and retrieved in a specific order. Most queues are categorized as one of the following:  First-in, first-out (FIFO) queue: Elements are inserted at the queue’s tail and removed at the queue’s head.  Last-in, first-out (LIFO) queue: Elements are inserted and removed at one end of the queue such that the last element inserted is the first element retrieved. This kind of queue behaves as a stack.  Priority queue: Elements are inserted according to their natural ordering, or according to a comparator that is supplied to the queue implementation. Queue, whose generic type is Queue, extends Collection, redeclaring add() to adjust its contract (insert the specified element into this queue if it is possible to do so immediately without violating capacity restrictions), and inheriting the other methods from Collection. Table 8–4 describes add() and the other Queue-specific methods. Table 8–4. Queue-specific Methods Method Description boolean add(E e) Insert element e into this queue if it is possible to do so immediately without violating capacity restrictions. Return true on success; otherwise, throw IllegalStateException when the element cannot be added at this time because of a capacity restriction. This method also throws ClassCastException when e’s class prevents e from being added to this queue, NullPointerException when e contains the null reference and this queue does not permit null elements to be added, and IllegalArgumentException when some property of e prevents it from being added to this queue. CHAPTER 8: Discovering the Collections Framework 347 Method Description E element() Return but do not also remove the element at the head of this queue. This method throws NoSuchElementException when this queue is empty. boolean offer(E e) Insert element e into this queue if it is possible to do so immediately without violating capacity restrictions. Return true on success; otherwise, return false when the element cannot be added at this time because of a capacity restriction. This method throws ClassCastException when e’s class prevents e from being added to this queue, NullPointerException when e contains the null reference and this queue does not permit null elements to be added, and IllegalArgumentException when some property of e prevents it from being added to this queue. E peek() Return but do not also remove the element at the head of this queue. This method returns null when this queue is empty. E poll() Return and also remove the element at the head of this queue. This method returns null when this queue is empty. E remove() Return and also remove the element at the head of this queue. This method throws NoSuchElementException when this queue is empty. This is the only difference between remove() and poll(). Table 8–4 reveals two sets of methods: in one set, a method (such as add()) throws an exception when an operation fails; in the other set, a method (such as offer()) returns a special value (false or null) in the presence of failure. The methods that return a special value are useful in the context of capacity-restricted Queue implementations where failure is a normal occurrence. Note: The offer() method is generally preferable to add() when using a capacity-restricted queue because offer() does not throw IllegalStateException. Java supplies many Queue implementation classes, where most of these classes are members of the java.util.concurrent package: LinkedBlockingQueue and SynchronousQueue are examples. In contrast, the java.util package provides LinkedList and PriorityQueue as its Queue implementation classes. CAUTION: Many Queue implementation classes do not allow null elements to be added. However, some classes (such as LinkedList) permit null elements. You should avoid adding a null element because null is used as a special return value by the peek() and poll() methods to indicate that a queue is empty. CHAPTER 8: Discovering the Collections Framework 348 PriorityQueue The PriorityQueue class provides an implementation of a priority queue, which is a queue that orders its elements according to their natural ordering or by a comparator provided when the queue is instantiated. Priority queues do not permit null elements, and do not permit insertion of non-Comparable objects when relying on natural ordering. The element at the head of the priority queue is the least element with respect to the specified ordering. If multiple elements are tied for least element, one of those elements is arbitrarily chosen as the least element. Similarly, the element at the tail of the priority queue is the greatest element, which is arbitrarily chosen when there is a tie. Priority queues are unbounded, but have a capacity that governs the size of the internal array that is used to store the priority queue’s elements. The capacity value is at least as large as the queue’s length, and grows automatically as elements are added to the priority queue. PriorityQueue (whose generic type is PriorityQueue) supplies six constructors:  PriorityQueue() creates a PriorityQueue instance with an initial capacity of 11 elements, and which orders its elements according to their natural ordering.  PriorityQueue(Collection collection) creates a PriorityQueue instance containing collection’s elements. If collection is a SortedSet or PriorityQueue instance, this priority queue will be ordered according to the same ordering. Otherwise, this priority queue will be ordered according to the natural ordering of its elements. This constructor throws ClassCastException when collection’s elements cannot be compared to one another according to the priority queue’s ordering, and NullPointerException when collection or any of its elements contain the null reference.  PriorityQueue(int initialCapacity) creates a PriorityQueue instance with the specified initialCapacity, and which orders its elements according to their natural ordering. This constructor throws IllegalArgumentException when initialCapacity is less than 1.  PriorityQueue(int initialCapacity, Comparator comparator) creates a PriorityQueue instance with the specified initialCapacity, and which orders its elements according to the specified comparator. Natural ordering is used when comparator contains the null reference. This constructor throws IllegalArgumentException when initialCapacity is less than 1. CHAPTER 8: Discovering the Collections Framework 349  PriorityQueue(PriorityQueue priorityQueue) creates a PriorityQueue instance containing priorityQueue’s elements. This priority queue will be ordered according to the same ordering as priorityQueue. This constructor throws ClassCastException when priorityQueue’s elements cannot be compared to one another according to priorityQueue’s ordering, and NullPointerException when priorityQueue or any of its elements contains the null reference.  PriorityQueue(SortedSet sortedSet) creates a PriorityQueue instance containing sortedSet’s elements. This priority queue will be ordered according to the same ordering as sortedSet. This constructor throws ClassCastException when sortedSet’s elements cannot be compared to one another according to sortedSet’s ordering, and NullPointerException when sortedSet or any of its elements contains the null reference. Listing 8–18 demonstrates a priority queue. Listing 8–18. Adding randomly generated integers to a priority queue import java.util.PriorityQueue; import java.util.Queue; public class PriorityQueueDemo { public static void main(String[] args) { Queue qi = new PriorityQueue(); for (int i = 0; i < 15; i++) qi.add((int) (Math.random()*100)); while (!qi.isEmpty()) System.out.print(qi.poll() + " "); System.out.println(); } } After creating a priority queue, the main thread adds 15 randomly generated integers (ranging from 0 through 99) to this queue. It then enters a while loop that repeatedly polls the priority queue for the next element and outputs that element until the queue is empty. When you run this application, it outputs a line of 15 integers in ascending numerical order from left to right. For example, I observed the following output from one run: 5 10 12 13 33 36 40 41 44 55 62 63 64 71 91 Because poll() returns null when there are no more elements, I could have coded this loop as follows: Integer i; while ((i = qi.poll()) != null) System.out.print(i + " "); Suppose you want to reverse the order of the previous example’s output so that the largest element appears on the left and the smallest element appears on the right. As CHAPTER 8: Discovering the Collections Framework 350 Listing 8–19 demonstrates, you can achieve this task by passing a comparator to the appropriate PriorityQueue constructor. Listing 8–19. Using a comparator with a priority queue import java.util.Comparator; import java.util.PriorityQueue; import java.util.Queue; public class PriorityQueueDemo { final static int NELEM = 15; public static void main(String[] args) { Comparator cmp; cmp = new Comparator() { public int compare(Integer e1, Integer e2) { return e2-e1; } }; Queue qi = new PriorityQueue(NELEM, cmp); for (int i = 0; i < NELEM; i++) qi.add((int) (Math.random()*100)); while (!qi.isEmpty()) System.out.print(qi.poll() + " "); System.out.println(); } } Listing 8–19 is similar to Listing 8–18, but there are some differences. First, I have declared an NELEM constant so that I can easily change both the priority queue’s initial capacity and the number of elements inserted into the priority queue by specifying the new value in one place. Second, Listing 8–19 declares and instantiates an anonymous class that implements Comparator. Its compareTo() method subtracts element e2 from element e1 to achieve descending numerical order. The compiler handles the task of unboxing e2 and e1 by converting e2-e1 to e2.intValue()-e1.intValue(). Finally, Listing 8–19 passes an initial capacity of NELEM elements and the instantiated comparator to the PriorityQueue(int initialCapacity, Comparator comparator) constructor. The priority queue will use this comparator to order these elements. Run this application and you will now see a single output line of 15 integers shown in descending numerical order from left to right. For example, I observed this output line: 91 90 80 72 71 69 67 65 53 38 33 25 8 8 7 CHAPTER 8: Discovering the Collections Framework 351 Map A map is a group of key/value pairs (also known as entries). Because the key identifies an entry, a map cannot contain duplicate keys. Furthermore, each key can map to at most one value. Maps are described by the Map interface, which has no parent interface, and whose generic type is Map (K is the key’s type; V is the value’s type). Table 8–5 describes Map’s methods. Table 8–5. Map Methods Method Description void clear() Remove all elements from this map, leaving it empty. This method throws UnsupportedOperationException when clear() is not supported. boolean containsKey (Object key) Return true when this map contains an entry for the specified key; otherwise, return false. This method throws ClassCastException when key is of an inappropriate type for this map, and NullPointerException when key contains the null reference and this map does not permit null keys. boolean containsValue(Object value) Return true when this map maps one or more keys to value. This method throws ClassCastException when value is of an inappropriate type for this map, and NullPointerException when value contains the null reference and this map does not permit null values. Set> entrySet() Return a Set view of the entries contained in this map. Because the view is backed by this map, changes that are made to the map are reflected in the set and vice versa. boolean equals(Object o) Compare o with this map for equality. Return true when o is also a map and the two maps represent the same entries; otherwise, return false. V get(Object key) Return the value to which key is mapped, or null when this map contains no entry for key. If this map permits null values, then a return value of null does not necessarily indicate that the map contains no entry for key; it is also possible that the map explicitly maps key to the null reference. The containsKey() method may be used to distinguish between these two cases. This method throws ClassCastException when key is of an inappropriate type for this map, and NullPointerException when key contains the null reference and this map does not permit null keys. CHAPTER 8: Discovering the Collections Framework 352 Method Description int hashCode() Return the hash code for this map. A map’s hash code is defined to be the sum of the hash codes for the entries in the map’s entrySet() view. boolean isEmpty() Return true when this map contains no entries; otherwise, return false. Set keySet() Return a Set view of the keys contained in this map. Because the view is backed by this map, changes that are made to the map are reflected in the set and vice versa. V put(K key,V value) Associate value with key in this map. If the map previously contained an entry for key, the old value is replaced by value. This method returns the previous value associated with key, or null when there was no entry for key. (The null return value can also indicate that the map previously associated the null reference with key, if the implementation supports null values.) This method throws UnsupportedOperationException when put() is not supported, ClassCastException when key’s or value’s class is not appropriate for this map, IllegalArgumentException when some property of key or value prevents it from being stored in this map, and NullPointerException when key or value contains the null reference and this map does not permit null keys or values. void putAll(Map m) Copy all of the entries from map m to this map. The effect of this call is equivalent to that of calling put(k, v) on this map once for each mapping from key k to value v in map m. This method throws UnsupportedOperationException when putAll() is not supported, ClassCastException when the class of a key or value in map m is not appropriate for this map, IllegalArgumentException when some property of a key or value in map m prevents it from being stored in this map, and NullPointerException when m contains the null reference or when m contains null keys or values and this map does not permit null keys or values. V remove(Object key) Remove key’s entry from this map if it is present. This method returns the value to which this map previously associated with key, or null when the map contained no mapping for key. If this map permits null values, then a return value of null does not necessarily indicate that the map contained no entry for key; it is also possible that the map explicitly mapped key to null. This map will not contain an entry for key once the call returns. This method throws UnsupportedOperationException when remove() is not supported, ClassCastException when the class of key is not appropriate for this map, and NullPointerException when key contains the null reference and this map does not permit null keys. CHAPTER 8: Discovering the Collections Framework 353 Method Description int size() Return the number of key/value entries in this map. If the map contains more than Integer.MAX_VALUE entries, this method returns Integer.MAX_VALUE. Collection values() Return a Collection view of the values contained in this map. Because the view is backed by this map, changes that are made to the map are reflected in the collection and vice versa. Unlike List, Set, and Queue, Map does not extend Collection. However, it is possible to view a map as a Collection instance by calling Map’s keySet(), values(), and entrySet() methods, which respectively return a Set of keys, a Collection of values, and a Set of key/value pair entries. NOTE: The values() method returns Collection instead of Set because multiple keys can map to the same value, and values() would then return multiple copies of the same value. The Collection views returned by these methods (recall that a Set is a Collection because Set extends Collection) provide the only means to iterate over a Map. For example, suppose you declare Listing 8–20’s Color enum with its three Color constants, RED, GREEN, and BLUE. Listing 8–20. A colorful enum enum Color { RED(255, 0, 0), GREEN(0, 255, 0), BLUE(0, 0, 255); private int r, g, b; private Color(int r, int g, int b) { this.r = r; this.g = g; this.b = b; } @Override public String toString() { return "r = " + r + ", g = " + g + ", b = " + b; } } Listing 8–21’s code fragment declares a map of String keys and Color values, adds several entries to the map, and iterates over the keys and values. Listing 8–21. Iterating over a map’s String-based keys and Color-based values Map colorMap = …; // … represents the creation of a Map implementation colorMap.put("red", Color.RED); colorMap.put("blue", Color.BLUE); CHAPTER 8: Discovering the Collections Framework 354 colorMap.put("green", Color.GREEN); colorMap.put("RED", Color.RED); for (String colorKey: colorMap.keySet()) System.out.println(colorKey); Collection colorValues = colorMap.values(); for (Iterator it = colorValues.iterator(); it.hasNext();) System.out.println(it.next()); When running this code fragment against a hashmap implementation (discussed later) of colorMap, you should observe output similar to the following: red blue green RED r = 255, g = 0, b = 0 r = 0, g = 0, b = 255 r = 0, g = 255, b = 0 r = 255, g = 0, b = 0 The first four output lines identify the map’s keys; the second four output lines identify the map’s values. The entrySet() method returns a Set of Map.Entry objects. Each of these objects describes a single entry as a key/value pair and is an instance of a class that implements the Map.Entry interface, where Entry is a nested interface of Map. Table 8–6 describes Map.Entry’s methods. NOTE: In Chapter 4, I mentioned that there does not appear to be a good reason to declare interfaces and classes in an interface’s body. Although I have yet to find a good reason to nest a class within an interface, Map.Entry proves that it is occasionally useful to nest an interface within another interface. Table 8–6. Map.Entry Methods Method Description boolean equals(Object o) Compare o with this entry for equality. Return true when o is also a map entry and the two entries have the same key and value. K getKey() Return this entry’s key. This method optionally throws IllegalStateException when this entry has previously been removed from the backing map. V getValue() Return this entry’s value. This method optionally throws IllegalStateException when this entry has previously been removed from the backing map. int hashCode() Return this entry’s hash code. This book was purchased by harshamv@variable3.comCHAPTER 8: Discovering the Collections Framework 355 Method Description V setValue(V value) Replace this entry’s value with value. The backing map is updated with the new value. This method throws UnsupportedOperationException when setValue() is not supported, ClassCastException when value’s class prevents it from being stored in the backing map, NullPointerException when value contains the null reference and the backing map does not permit null, IllegalArgumentException when some property of value prevents it from being stored in the backing map, and (optionally) IllegalStateException when this entry has previously been removed from the backing map. Continuing from the previous example, Listing 8–22 shows you how you might iterate over the map’s entries. Listing 8–22. Iterating over a map’s entries for (Map.Entry colorEntry: colorMap.entrySet()) System.out.println(colorEntry.getKey() + ": " + colorEntry.getValue()); When running Listing 8–22’s code fragment against the previously mentioned hashmap implementation, you would observe the following output: red: r = 255, g = 0, b = 0 blue: r = 0, g = 0, b = 255 green: r = 0, g = 255, b = 0 RED: r = 255, g = 0, b = 0 TreeMap The TreeMap class provides a map implementation that is based on a red-black tree. As a result, entries are stored in sorted order of their keys. However, accessing these entries is somewhat slower than with the other Map implementations (which are not sorted) because links must be traversed. NOTE: Check out Wikipedia’s “Red-black tree” entry (http://en.wikipedia.org/wiki/Red-black_tree) to learn about red-black trees. TreeMap supplies four constructors:  TreeMap() creates a new, empty tree map that is sorted according to the natural ordering of its keys. All keys inserted into the map must implement the Comparable interface.  TreeMap(Comparator comparator) creates a new, empty tree map that is sorted according to the specified comparator. Passing null to comparator implies that natural ordering will be used. CHAPTER 8: Discovering the Collections Framework 356  TreeMap(Map map) creates a new tree map containing map’s entries, sorted according to the natural ordering of its keys. All keys inserted into the new map must implement the Comparable interface. This constructor throws ClassCastException when map’s keys do not implement Comparable or are not mutually comparable, and NullPointerException when map contains the null reference.  TreeMap(SortedMap sortedMap) creates a new tree map containing the same entries and using the same ordering as sortedMap. (I discuss sorted maps later in this chapter.) This constructor throws NullPointerException when sortedMap contains the null reference. Listing 8–23 demonstrates a tree map. Listing 8–23. Sorting a map’s entries according to the natural ordering of their String-based keys import java.util.Map; import java.util.TreeMap; public class TreeMapDemo { public static void main(String[] args) { Map msi = new TreeMap(); String[] fruits = {"apples", "pears", "grapes", "bananas", "kiwis"}; int[] quantities = {10, 15, 8, 17, 30}; for (int i = 0; i < fruits.length; i++) msi.put(fruits[i], quantities[i]); for (Map.Entry entry: msi.entrySet()) System.out.println(entry.getKey() + ": " + entry.getValue()); } } When you run this application, it generates the following output: apples: 10 bananas: 17 grapes: 8 kiwis: 30 pears: 15 HashMap The HashMap class provides a map implementation that is based on a hashtable data structure. This implementation supports all Map operations, and permits null keys and null values. It makes no guarantees on the order in which entries are stored. A hashtable maps keys to integer values with the help of a hash function. Java provides this function in the form of Object’s hashCode() method, which classes override to provide appropriate hash codes. CHAPTER 8: Discovering the Collections Framework 357 A hash code identifies one of the hashtable’s array elements, which is known as a bucket or slot. For some hashtables, the bucket may store the value that is associated with the key. Figure 8–2 illustrates this kind of hashtable. Figure 8–2. A simple hashtable of buckets The hash function hashes Bob Doe to 0, which identifies the first bucket. This bucket contains ACCTS, which is Bob Doe’s employee type. The hash function also hashes John Doe and Sally Doe to 1 and 2 (respectively) whose buckets contain SALES. A perfect hash function hashes each key to a unique integer value. However, this ideal is very difficult to meet. In practice, some keys will hash to the same integer value. This nonunique mapping is referred to as a collision. To address collisions, most hashtables associate a linked list of entries with a bucket. Instead of containing a value, the bucket contains the address of the first node in the linked list, and each node contains one of the colliding entries. See Figure 8–3. Figure 8–3. A complex hashtable of buckets and linked lists (X indicates a null reference) When storing a value in a hashtable, the hashtable uses the hash function to hash the key to its hash code, and then searches the appropriate linked list to see if an entry with a matching key exists. If there is an entry, its value is updated with the new value. CHAPTER 8: Discovering the Collections Framework 358 Otherwise, a new node is created, populated with the key and value, and appended to the list. When retrieving a value from a hashtable, the hashtable uses the hash function to hash the key to its hash code, and then searches the appropriate linked list to see if an entry with a matching key exists. If there is an entry, its value is returned. Otherwise, the hashtable may return a special value to indicate that there is no entry, or it might throw an exception. The number of buckets is known as the hashtable’s capacity. The ratio of the number of stored entries divided by the number of buckets is known as the hashtable’s load factor. Choosing the right load factor is important for balancing performance with memory use:  As the load factor approaches 1, the probability of collisions and the cost of handling them (by searching lengthy linked lists) increase.  As the load factor approaches 0, the hashtable’s size in terms of number of buckets increases with little improvement in search cost.  For many hashtables, a load factor of 0.75 is close to optimal. This value is the default for HashMap’s hashtable implementation. HashMap supplies four constructors:  HashMap() creates a new, empty hashmap with an initial capacity of 16 and a load factor of 0.75.  HashMap(int initialCapacity) creates a new, empty hashmap with a capacity specified by initialCapacity and a load factor of 0.75. This constructor throws IllegalArgumentException when initialCapacity’s value is less than 0.  HashMap(int initialCapacity, float loadFactor) creates a new, empty hashmap with a capacity specified by initialCapacity and a load factor specified by loadFactor. This constructor throws IllegalArgumentException when initialCapacity is less than 0 or when loadFactor is less than or equal to 0.  HashMap(Map map) creates a new hashmap containing map’s entries. This constructor throws NullPointerException when map contains the null reference. Listing 8–24 demonstrates a hashmap. Listing 8–24. Using a hashmap to count command-line arguments import java.util.HashMap; import java.util.Map; public class HashMapDemo { public static void main(String[] args) { Map argMap = new HashMap(); for (String arg: args) CHAPTER 8: Discovering the Collections Framework 359 { Integer count = argMap.get(arg); argMap.put(arg, (count == null) ? 1 : count+1); } System.out.println(argMap); System.out.println("Number of distinct arguments = " + argMap.size()); } } HashMapDemo creates a hashmap of String keys and Integer values. Each key is one of the command-line arguments passed to this application, and its value is the number of occurrences of that argument on the command line. For example, java HashMapDemo how much wood could a woodchuck chuck if a woodchuck could chuck wood generates the following output: {wood=2, could=2, how=1, if=1, chuck=2, a=2, woodchuck=2, much=1} Number of distinct arguments = 8 Because the String class overrides equals() and hashCode(), Listing 8–24 can use String objects as keys in a hashmap. When you create a class whose instances are to be used as keys, you must ensure that you override both methods. Listing 8–10 showed you that a class’s overriding hashCode() method can call a reference field’s hashCode() method and return its value, provided that the class declares a single reference field (and no primitive type fields). More commonly, classes declare multiple fields, and a better implementation of the hashCode() method is required. The implementation should try to generate hash codes that minimize collisions. There is no rule on how to best implement hashCode(), and various algorithms (recipes for accomplishing tasks) have been created. My favorite algorithm appears in Effective Java, Second Edition, by Joshua Bloch (Addison-Wesley, 2008; ISBN: 0321356683). The following algorithm, which assumes the existence of an arbitrary class that is referred to as X, closely follows Bloch’s algorithm, but is not identical: 1. Initialize int variable hashCode (the name is arbitrary) to an arbitrary nonzero integer value, such as 19. This variable is initialized to a nonzero value to ensure that it takes into account any initial fields whose hash codes are zeros. If you initialize hashCode to 0, the final hash code will be unaffected by such fields and you run the risk of increased collisions. 2. For each field f that is also used in X’s equals() method, calculate f’s hash code and assign it to int variable hc as follows: a. If f is of Boolean type, calculate hc = f?1:0. b. If f is of byte integer, character, integer, or short integer type, calculate hc = (int) f. The integer value is the hash code. CHAPTER 8: Discovering the Collections Framework 360 c. If f is of long integer type, calculate hc = (int) (f^(f>>>32)). This expression exclusive ORs the long integer’s least significant 32 bits with its most significant 32 bits. d. If f is of type floating-point, calculate hc = Float.floatToIntBits(f). This method takes +infinity, -infinity, and NaN into account. e. If f is of type double precision floating-point, calculate long l = Double.doubleToLongBits(f); hc = (int) (l^(l>>>32)). f. If f is a reference field with a null reference, calculate hc = 0. g. If f is a reference field with a nonnull reference, and if X’s equals() method compares the field by recursively calling equals() (as in Listing 8–17’s Employee class), calculate hc = f.hashCode(). However, if equals() employs a more complex comparison, create a canonical (simplest possible) representation of the field and call hashCode() on this representation. h. If f is an array, treat each element as a separate field by applying this algorithm recursively and combining the hc values as shown in the next step. 3. Combine hc with hashCode as follows: hashCode = hashCode*31+hc. Multiplying hashCode by 31 makes the resulting hash value dependent on the order in which fields appear in the class, which improves the hash value when a class contains multiple fields that are similar (several ints, for example). I chose 31 to be consistent with the String class’s hashCode() method. 4. Return hashCode from hashCode(). TIP: Instead of using this or another algorithm to create a hash code, you might find it easier to work with the HashCodeBuilder class (see http://commons.apache.org/lang/api- 2.4/org/apache/commons/lang/builder/HashCodeBuilder.html for an explanation of this class). This class, which follows Bloch’s rules, is part of the Apache Commons Lang component, which you can download from http://commons.apache.org/lang/. In Chapter 3, Listing 3-9’s Point class overrides equals() but does not override hashCode(). Listing 3-11 presents a small code fragment that must be appended to Point’s main() method to demonstrate the problem of not overriding hashCode(). I restate this problem here: Although objects p1 and Point(10, 20) are logically equivalent, these objects have different hash codes, resulting in each object referring to a different entry in the hashmap. If an object is not stored (via put()) in that entry, get() returns null. CHAPTER 8: Discovering the Collections Framework 361 Listing 8–25 modifies Listing 3-9’s Point class by declaring a hashCode() method. This method uses the aforementioned algorithm to ensure that logically equivalent Point objects hash to the same entry. Listing 8–25. Overriding hashCode() to return proper hash codes for Point objects import java.util.HashMap; import java.util.Map; class Point { private int x, y; Point(int x, int y) { this.x = x; this.y = y; } int getX() { return x; } int getY() { return y; } @Override public boolean equals(Object o) { if (!(o instanceof Point)) return false; Point p = (Point) o; return p.x == x && p.y == y; } @Override public int hashCode() { int hashCode = 19; int hc = x; hashCode = hashCode*31+hc; hc = y; hashCode = hashCode*31+hc; return hc; } public static void main(String[] args) { Point p1 = new Point(10, 20); Point p2 = new Point(20, 30); Point p3 = new Point(10, 20); // Test reflexivity System.out.println(p1.equals(p1)); // Output: true // Test symmetry System.out.println(p1.equals(p2)); // Output: false System.out.println(p2.equals(p1)); // Output: false // Test transitivity System.out.println(p2.equals(p3)); // Output: false System.out.println(p1.equals(p3)); // Output: true // Test nullability CHAPTER 8: Discovering the Collections Framework 362 System.out.println(p1.equals(null)); // Output: false // Extra test to further prove the instanceof operator's usefulness. System.out.println(p1.equals("abc")); // Output: false Map map = new HashMap(); map.put(p1, "first point"); System.out.println(map.get(p1)); // Output: first point System.out.println(map.get(new Point(10, 20))); // Output: null } } The hashCode() method is a little verbose in that it assigns each of x and y to local variable hc, rather than directly using these fields in the hash code calculation. However, I decided to follow this approach to more closely mirror the hash code algorithm. When you run this application, its last two lines of output are of the most interest. Instead of presenting first point followed by null on two separate lines, the application now correctly presents first point followed by first point on these lines. NOTE: LinkedHashMap is a subclass of HashMap that uses a linked list to store its entries. As a result, LinkedHashMap’s iterator returns entries in the order in which they were inserted. For example, if Listing 8–24 had specified Map argMap = new LinkedHashMap();, the application’s output for java HashMapDemo how much wood could a woodchuck chuck if a woodchuck could chuck wood would have been {how=1, much=1, wood=2, could=2, a=2, woodchuck=2, chuck=2, if=1} followed by Number of distinct arguments = 8. IdentityHashMap The IdentityHashMap class provides a Map implementation that uses reference equality (==) instead of object equality (equals()) when comparing keys and values. This is an intentional violation of Map’s general contract, which mandates the use of equals() when comparing elements. IdentityHashMap obtains hash codes via System’s static int identityHashCode(Object x) method instead of via each key’s hashCode() method. identityHashCode() returns the same hash code for x as returned by Object’s hashCode() method, whether or not x’s class overrides hashCode(). The hash code for the null reference is zero. These characteristics give IdentityHashMap a performance advantage over other Map implementations. Also, IdentityHashMap supports mutable keys (objects used as keys and whose hash codes change when their field values change while in the map). Listing 8–26 contrasts IdentityHashMap with HashMap where mutable keys are concerned. Listing 8–26. Contrasting IdentityHashMap with HashMap in a mutable key context import java.util.IdentityHashMap; import java.util.HashMap; import java.util.Map; CHAPTER 8: Discovering the Collections Framework 363 public class IdentityHashMapDemo { public static void main(String[] args) { Map map1 = new IdentityHashMap(); Map map2 = new HashMap(); Employee e1 = new Employee("John Doe", 28); map1.put(e1, "SALES"); System.out.println(map1); Employee e2 = new Employee("Jane Doe", 26); map2.put(e2, "MGMT"); System.out.println(map2); System.out.println("map1 contains key e1 = " + map1.containsKey(e1)); System.out.println("map2 contains key e2 = " + map2.containsKey(e2)); e1.setAge(29); e2.setAge(27); System.out.println(map1); System.out.println(map2); System.out.println("map1 contains key e1 = " + map1.containsKey(e1)); System.out.println("map2 contains key e2 = " + map2.containsKey(e2)); } } class Employee { private String name; private int age; Employee(String name, int age) { this.name = name; this.age = age; } @Override public boolean equals(Object o) { if (!(o instanceof Employee)) return false; Employee e = (Employee) o; return e.name.equals(name) && e.age == age; } @Override public int hashCode() { int hashCode = 19; hashCode = hashCode*31+name.hashCode(); hashCode = hashCode*31+age; return hashCode; } void setAge(int age) { this.age = age; } void setName(String name) { this.name = name; } @Override public String toString() CHAPTER 8: Discovering the Collections Framework 364 { return name + " " + age; } } Listing 8–26’s main() method creates IdentityHashMap and HashMap instances that each store an entry consisting of an Employee key and a String value. Because Employee instances are mutable (because of setAge() and setName()), main() changes their ages while these keys are stored in their maps. These changes result in the following output: {John Doe 28=SALES} {Jane Doe 26=MGMT} map1 contains key e1 = true map2 contains key e2 = true {John Doe 29=SALES} {Jane Doe 27=MGMT} map1 contains key e1 = true map2 contains key e2 = false The last four lines show that the changed entries remain in their maps. However, map2’s containsKey() method reports that its HashMap instance no longer contains its Employee key (which should be Jane Doe 27), whereas map1’s containsKey() method reports that its IdentityHashMap instance still contains its Employee key, which is now John Doe 29. NOTE: IdentityHashMap’s documentation states that “a typical use of this class is topology- preserving object graph transformations, such as serialization or deep copying.” (I discuss serialization in Chapter 10.) It also states that “another typical use of this class is to maintain proxy objects.” Also, developers responding to stackoverflow’s “Use Cases for Identity HashMap” topic (http://stackoverflow.com/questions/838528/use-cases-for-identity- hashmap) mention that it is much faster to use IdentityHashMap than HashMap when the keys are Class objects. WeakHashMap The WeakHashMap class provides a Map implementation that is based on weakly reachable keys. Because each key object is stored indirectly as the referent of a weak reference, the key is automatically removed from the map only after the garbage collector clears all weak references to the key (inside and outside of the map). NOTE: Check out Chapter 6’s “References API” section to learn about weakly reachable and weak references. In contrast, value objects are stored via strong references. These objects should not strongly refer to their own keys, either directly or indirectly, because doing so prevents their associated keys from being discarded. When a key is removed from a map, its associated value object is also removed. This book was purchased by harshamv@variable3.comCHAPTER 8: Discovering the Collections Framework 365 Listing 8–27 provides a simple demonstration of the WeakHashMap class. Listing 8–27. Automatically removing a String value object from a weak hashmap when the strong reference to its associated LargeObject key object is nullified import java.util.Map; import java.util.WeakHashMap; class LargeObject { private byte[] memory = new byte[1024*1024*50]; // 50 megabytes } public class WeakHashMapDemo { public static void main(String[] args) { Map map = new WeakHashMap(); LargeObject lo = new LargeObject(); map.put(lo, "Large Object"); System.out.println(map); lo = null; while (!map.isEmpty()) { System.gc(); new LargeObject(); } System.out.println(map); } } Listing 8–27’s main() method stores a 50MB LargeObject key and a String value in the weak hashmap, and then removes the key’s strong reference by assigning null to lo. main() next enters a while loop that executes until the map is empty (map.isEmpty() returns true). Each loop iteration begins with a System.gc() method call, which may or may not cause a garbage collection to take place (depending upon platform). To encourage a garbage collection, the iteration then creates a LargeObject object and throws away its reference. This activity should eventually cause the garbage collector to run and remove the map’s solitary entry. When I run this application on my Windows XP platform, I observe the following output—you might need to modify the code if you find that the application is in an infinite loop: {LargeObject@addbf1=Large Object} {} NOTE: WeakHashMap is useful for avoiding memory leaks, as explained in Brian Goetz’s article “Java Theory and Practice: Plugging Memory Leaks with Weak References” (http://www.ibm.com/developerworks/java/library/j-jtp11225/). CHAPTER 8: Discovering the Collections Framework 366 EnumMap The EnumMap class provides a Map implementation whose keys are the members of the same enum. Null keys are not permitted; any attempt to store a null key results in a thrown NullPointerException. Because an enum map is represented internally as an array, an enum map approaches an array in terms of performance. EnumMap supplies the following constructors:  EnumMap(Class keyType) creates an empty enum map with the specified keyType. This constructor throws NullPointerException when keyType contains the null reference.  EnumMap(EnumMap map) creates an enum map with the same key type as map, and with map’s entries. This constructor throws NullPointerException when map contains the null reference.  EnumMap(Map map) creates an enum map initialized with map’s entries. If map is an EnumMap instance, this constructor behaves like the previous constructor. Otherwise, map must contain at least one entry in order to determine the new enum map’s key type. This constructor throws NullPointerException when map contains the null reference, and IllegalArgumentException when map is not an EnumMap instance and is empty. Listing 8–28 demonstrates EnumMap. Listing 8–28. An enum map of Coin constants import java.util.EnumMap; import java.util.Map; enum Coin { PENNY, NICKEL, DIME, QUARTER } public class EnumMapDemo { public static void main(String[] args) { Map map = new EnumMap(Coin.class); map.put(Coin.PENNY, 1); map.put(Coin.NICKEL, 5); map.put(Coin.DIME, 10); map.put(Coin.QUARTER, 25); System.out.println(map); Map mapCopy = new EnumMap(map); System.out.println(mapCopy); } } When you run this application, it generates the following output: {PENNY=1, NICKEL=5, DIME=10, QUARTER=25} {PENNY=1, NICKEL=5, DIME=10, QUARTER=25} CHAPTER 8: Discovering the Collections Framework 367 SortedMap TreeMap is an example of a sorted map, which is a map that maintains its entries in ascending order, sorted according to the keys’ natural ordering or according to a comparator that is supplied when the sorted map is created. Sorted maps are described by the SortedMap interface. SortedMap, whose generic type is SortedMap, extends Map. With two exceptions, the methods it inherits from Map behave identically on sorted maps as on other maps:  The Iterator instance returned by the iterator() method on any of the sorted map’s Collection views traverses the collections in order.  The arrays returned by the Collection views’ toArray() methods contain the keys, values, or entries in order. NOTE: Although not guaranteed, the toString() methods of the Collection views of SortedSet implementations in the collections framework (such as TreeMap) return a string containing all of the view’s elements in order. SortedMap’s documentation requires that an implementation must provide the four standard constructors that I presented in my discussion of TreeMap. Furthermore, implementations of this interface must implement the methods that are described in Table 8–7. Table 8–7. SortedMap-specific Methods Method Description Comparator comparator() Return the comparator used to order the keys in this map, or null when this map uses the natural ordering of its keys. Set> entrySet() Return a Set view of the mappings contained in this map. The set’s iterator returns these entries in ascending key order. Because the view is backed by this map, changes that are made to the map are reflected in the set and vice versa. K firstKey() Return the first (lowest) key currently in this map, or throw a NoSuchElementException instance when this map is empty. SortedMap headMap (K toKey) Return a view of that portion of this map whose keys are strictly less than toKey. Because the returned map is backed by this map, changes in the returned map are reflected in this map and vice versa. The returned map supports all optional map operations that this map supports. This method throws ClassCastException when toKey is not compatible with this map’s comparator (or, when the map has no comparator, when toKey does not implement Comparable), NullPointerException when toKey is null and this map does not permit null keys, and IllegalArgumentException when this map has a restricted range and toKey lies outside of this range’s bounds. CHAPTER 8: Discovering the Collections Framework 368 Method Description Set keySet() Return a Set view of the keys contained in this map. The set’s iterator returns the keys in ascending order. Because the view is backed by this map, changes that are made to the map are reflected in the set and vice versa. K lastKey() Return the last (highest) key currently in this map, or throw a NoSuchElementException instance when this map is empty. SortedMap subMap (K fromKey, K toKey) Return a view of the portion of this map whose keys range from fromKey, inclusive, to toKey, exclusive. (When fromKey and toKey are equal, the returned map is empty.) Because the returned map is backed by this map, changes in the returned map are reflected in this map and vice versa. The returned map supports all optional map operations that this map supports. This method throws ClassCastException when fromKey and toKey cannot be compared to one another using this map’s comparator (or, when the map has no comparator, using natural ordering), NullPointerException when fromKey or toKey is null and this map does not permit null keys, and IllegalArgumentException when fromKey is greater than toKey or when this map has a restricted range and fromKey or toKey lies outside of this range’s bounds. SortedMap tailMap (K fromKey) Return a view of that portion of this map whose keys are greater than or equal to fromKey. Because the returned map is backed by this map, changes in the returned map are reflected in this map and vice versa. The returned map supports all optional map operations that this map supports. This method throws ClassCastException when fromKey is not compatible with this map’s comparator (or, when the map has no comparator, when fromKey does not implement Comparable), NullPointerException when fromKey is null and this map does not permit null elements, and IllegalArgumentException when this map has a restricted range and fromKey lies outside of the range’s bounds. Collection values() Return a Collection view of the values contained in this map. The collection’s iterator returns the values in ascending order of the corresponding keys. Because the collection is backed by the map, changes that are made to the map are reflected in the collection and vice versa. Listing 8–29 demonstrates a sorted map based on a tree map. Listing 8–29. A sorted map of office supply names and quantities import java.util.Comparator; import java.util.SortedMap; import java.util.TreeMap; public class SortedMapDemo CHAPTER 8: Discovering the Collections Framework 369 { public static void main(String[] args) { SortedMap smsi = new TreeMap(); String[] officeSupplies = { "pen", "pencil", "legal pad", "CD", "paper" }; int[] quantities = { 20, 30, 5, 10, 20 }; for (int i = 0; i < officeSupplies.length; i++) smsi.put(officeSupplies[i], quantities[i]); System.out.println(smsi); System.out.println(smsi.headMap("pencil")); System.out.println(smsi.headMap("paper")); SortedMap smsiCopy; Comparator cmp; cmp = new Comparator() { public int compare(String key1, String key2) { return key2.compareTo(key1); // descending order } }; smsiCopy = new TreeMap(cmp); smsiCopy.putAll(smsi); System.out.println(smsiCopy); } } When you run this application, it generates the following output: {CD=10, legal pad=5, paper=20, pen=20, pencil=30} {CD=10, legal pad=5, paper=20, pen=20} {CD=10, legal pad=5} {pencil=30, pen=20, paper=20, legal pad=5, CD=10} Utilities The collections framework would not be complete without its Arrays and Collections utility classes. Each class supplies various utility (static) methods that implement useful algorithms in the contexts of arrays and collections. Following is a sampling of the Arrays class’s array-oriented utility methods:  static List asList(T... array) returns a fixed-size list backed by the specified array. (Changes to the returned list “write through” to the array.) For example, List birds = Arrays.asList("Robin", "Oriole", "Bluejay"); converts the three- element array of Strings (recall that a variable sequence of arguments is implemented as an array) to a List whose reference is assigned to birds. CHAPTER 8: Discovering the Collections Framework 370  static int binarySearch(int[] array, int key) searches array for entry key using the binary search algorithm (explained following this list). The array must be sorted before calling this method; otherwise, the results are undefined. This method returns the index of the search key, if it is contained in the array; otherwise, (-(insertion point) - 1) is returned. The insertion point is the point at which key would be inserted into the array (the index of the first element greater than key, or array.length if all elements in the array are less than key) and guarantees that the return value will be greater than or equal to 0 if and only if key is found. For example, Arrays.binarySearch(new String[] {"Robin", "Oriole", "Bluejay"}, "Oriole") returns 1, "Oriole"’s index.  static void fill(char[] array, char character) stores character in each element of the specified character array. For example, Arrays.fill(screen[i], ' '); fills the ith row of a 2D screen array with spaces.  static void sort(long[] array) sorts the elements in the long integer array into ascending numerical order; for example, long lArray = new long[] { 20000L, 89L, 66L, 33L}; Arrays.sort(lArray);.  static void sort(T[] array, Comparator comparator) sorts the elements in array using comparator to order them. For example, when given Comparator cmp = new Comparator() { public int compare(String e1, String e2) { return e2.compareTo(e1); } }; String[] innerPlanets = { "Mercury", "Venus", "Earth", "Mars" };, Arrays.sort(innerPlanets, cmp); uses cmp to help in sorting innerPlanets into descending order of its elements: Venus, Mercury, Mars, Earth is the result. There are two common algorithms for searching an array for a specific element. Linear search searches the array element by element from index 0 to the index of the searched for element or the end of the array. On average, half of the elements must be searched; larger arrays take longer to search. However, the arrays do not need to be sorted. In contrast, binary search searches ordered array a’s n items for element e in a much faster amount of time. It works by recursively performing the following steps: 1. Set low index to 0. 2. Set high index to n-1. 3. If low index > high index, then Print “Unable to find ” e. End. 4. Set middle index to (low index+high index)/2. 5. If e > a[middle index], then set low index to middle index+1. Go to 3. 6. If e < a[middle index], then set high index to middle index-1. Go to 3. CHAPTER 8: Discovering the Collections Framework 371 7. Print “Found ” e “at index ” middle index. The algorithm is similar to optimally looking for a name in a phone book. Start by opening the book to the exact middle. If the name is not on that page, proceed to open the book to the exact middle of the first half or the second half, depending on in which half the name occurs. Repeat until you find the name (or not). Applying a linear search to 4,000,000,000 elements results in approximately 2,000,000,000 comparisons (on average), which takes time. In contrast, applying a binary search to 4,000,000,000 elements results in a maximum of 32 comparisons. This is why Arrays contains binarySearch() methods and not also linearSearch() methods. Following is a sampling of the Collections class’s collection-oriented utility methods:  static > T min(Collection collection) returns the minimum element of collection collection according to the natural ordering of its elements. For example, System.out.println(Collections.min(Arrays.asList(10, 3, 18, 25))); outputs 3. All of collection’s elements must implement the Comparable interface. Furthermore, all elements must be mutually comparable. This method throws NoSuchElementException when collection is empty.  static void reverse(List list) reverses the order of list’s elements. For example, List birds = Arrays.asList("Robin", "Oriole", "Bluejay"); Collections.reverse(birds); System.out.println(birds); results in [Bluejay, Oriole, Robin] as the output.  static List singletonList(T o) returns an immutable list containing only object o. For example, list.removeAll(Collections.singletonList(null)); removes all null elements from list.  static Set synchronizedSet(Set set) returns a synchronized (thread-safe) set backed by the specified set; for example, Set ss = Collections.synchronizedSet(new HashSet());. In order to guarantee serial access, it is critical that all access to the backing set is accomplished through the returned set.  static Map unmodifiableMap(Map map) returns an unmodifiable view of the specified map; for example, Map msi = Collections.synchronizedMap(new HashMap());. Query operations on the returned map “read through” to the specified map, and attempts to modify the returned map, whether direct or via its collection views, result in an UnsupportedOperationException. CHAPTER 8: Discovering the Collections Framework 372 NOTE: For performance reasons, collections implementations are unsynchronized— unsynchronized collections have better performance than synchronized collections. To use a collection in a multithreaded context, however, you need to obtain a synchronized version of that collection. You obtain that version by calling a method such as synchronizedSet(). Classic Collections Classes Java version 1.2 introduced the collections framework. Prior to the framework’s inclusion in Java, developers had two choices where collections were concerned: create their own frameworks, or use the Vector, Enumeration, Stack, Dictionary, Hashtable, Properties, and BitSet types, which were introduced by Java version 1.0. Vector is a concrete class that describes a growable array, much like ArrayList. Unlike an ArrayList instance, a Vector instance is synchronized. Vector has been generified and also retrofitted to support the collections framework, which makes statements such as List list = new Vector(); legal. The collections framework provides Iterator for iterating over a collection’s elements. In contrast, Vector’s elements() method returns an instance of a class that implements the Enumeration interface for enumerating (iterating over and returning) a Vector instance’s elements via Enumeration’s hasMoreElements() and nextElement() methods. Vector is subclassed by the concrete Stack class, which represents a LIFO data structure. Stack provides an E push(E item) method for pushing an object onto the stack, an E pop() method for popping an item off the top of the stack, and a few other methods, such as boolean empty() for determining whether or not the stack is empty. Stack is a good example of bad API design. By inheriting from Vector, it is possible to call Vector’s void add(int index, E element) method to add an element anywhere you wish, and violate a Stack instance’s integrity. In hindsight, Stack should have used composition in its design: use a Vector instance to store a Stack instance’s elements. Dictionary is an abstract superclass for subclasses that map keys to values. The concrete Hashtable class is Dictionary’s only subclass. As with Vector, HashTable instances are synchronized, HashTable has been generified, and HashTable has been retrofitted to support the collections framework. Hashtable is subclassed by Properties, a concrete class representing a persistent set of properties (String-based key/value pairs that identify application settings). Properties provides Object setProperty(String key, String value) for storing a property, and public String getProperty(String key) for returning a property’s value. CHAPTER 8: Discovering the Collections Framework 373 NOTE: Applications use properties for various purposes. For example, if your application has a graphical user interface, you might persist its main window’s screen location and size to a file via a Properties object so that the application can restore the window’s location and size when it next runs. Properties is another good example of bad API design. By inheriting from Hashtable, you can call Hashtable’s V put(K key, V value) method to store an entry with a non- String key and/or a non-String value. In hindsight, Properties should have leveraged composition: store a Properties instance’s elements in a Hashtable instance. NOTE: Chapter 3 discusses wrapper classes, which is how Stack and Properties should have been implemented. Finally, BitSet is a concrete class that describes a variable-length set of bits. This class’s ability to represent bitsets of arbitrary length contrasts with the previously described integer-based, fixed-length bitset that is limited to a maximum number of members: 32 members for an int-based bitset, or 64 members for a long-based bitset. BitSet provides a pair of constructors for initializing a BitSet instance: BitSet() initializes the instance to initially store an implementation-dependent number of bits, whereas BitSet(int nbits) initializes the instance to initially store nbits bits. BitSet also provides various methods, including the following:  void and(BitSet bs) bitwise ANDs this bitset with bs. This bitset is modified such that a bit is set to 1 when it and the bit at the same position in bs are 1.  void andNot(BitSet bs) sets all of the bits in this bitset to 0 whose corresponding bits are set to 1 in bs.  void clear() sets all of the bits in this bitset to 0.  Object clone() clones this bitset to produce a new bitset. The clone has exactly the same bits set to one as this bitset.  boolean get(int bitIndex) returns the value of this bitset’s bit, as a Boolean true/false value (true for 1, false for 0) at the zero-based bitIndex. This method throws IndexOutOfBoundsException when bitIndex is less than 0.  int length() returns the “logical size” of this bitset, which is the index of the highest 1 bit plus 1, or 0 if this bitset contains no 1 bits.  void or(BitSet bs) bitwise inclusive ORs this bitset with bs. This bitset is modified such that a bit is set to 1 when it or the bit at the same position in bs is 1, or when both bits are 1. CHAPTER 8: Discovering the Collections Framework 374  void set(int bitIndex, boolean value) sets the bit at the zero- based bitIndex to value (true is converted to 1; false is converted to 0). This method throws IndexOutOfBoundsException when bitIndex is less than 0.  int size() returns the number of bits that are being used by this bitset to represent bit values.  String toString() returns a string representation of this bitset in terms of the positions of bits that are 1; for example, {4, 5, 9, 10}.  void xor(BitSet set) bitwise exclusive ORs this bitset with bs. This bitset is modified such that a bit is set to 1 when either it or the bit at the same position in bs (but not both) is 1. Listing 8–30 presents an application that demonstrates some of these methods, and gives you more insight into how the bitwise AND (&), bitwise inclusive OR (|), and bitwise exclusive OR (^) operators work. Listing 8–30. Working with variable-length bitsets import java.util.BitSet; public class BitSetDemo { public static void main(String[] args) { BitSet bs1 = new BitSet(); bs1.set(4, true); bs1.set(5, true); bs1.set(9, true); bs1.set(10, true); BitSet bsTemp = (BitSet) bs1.clone(); dumpBitset(" ", bs1); BitSet bs2 = new BitSet(); bs2.set(4, true); bs2.set(6, true); bs2.set(7, true); bs2.set(9, true); dumpBitset(" ", bs2); bs1.and(bs2); dumpSeparator(Math.min(bs1.size(), 16)); dumpBitset("AND (&) ", bs1); System.out.println(); bs1 = bsTemp; dumpBitset(" ", bs1); dumpBitset(" ", bs2); bsTemp = (BitSet) bs1.clone(); bs1.or(bs2); dumpSeparator(Math.min(bs1.size(), 16)); dumpBitset("OR (|) ", bs1); System.out.println(); bs1 = bsTemp; dumpBitset(" ", bs1); dumpBitset(" ", bs2); bsTemp = (BitSet) bs1.clone(); This book was purchased by harshamv@variable3.comCHAPTER 8: Discovering the Collections Framework 375 bs1.xor(bs2); dumpSeparator(Math.min(bs1.size(), 16)); dumpBitset("XOR (^) ", bs1); } static void dumpBitset(String preamble, BitSet bs) { System.out.print(preamble); for (int i = 0; i < Math.min(bs.size(), 16); i++) System.out.print(bs.get(i) ? "1" : "0"); System.out.print(" size(" + bs.size() + "), length(" + bs.length() + ")"); System.out.println(); } static void dumpSeparator(int len) { System.out.print(" "); for (int i = 0; i < len; i++) System.out.print("-"); System.out.println(); } } Why did I specify Math.min(bs.size(), 16) in dumpBitset(), and pass a similar expression to dumpSeparator()? I wanted to display exactly 16 bits and 16 dashes (for aesthetics), and needed to account for a bitset’s size being less than 16. Although this does not happen with the JDK’s BitSet class, it might happen with a non-JDK variant. When you run this application, it generates the following output: 0000110001100000 size(64), length(11) 0000101101000000 size(64), length(10) ---------------- AND (&) 0000100001000000 size(64), length(10) 0000110001100000 size(64), length(11) 0000101101000000 size(64), length(10) ---------------- OR (|) 0000111101100000 size(64), length(11) 0000110001100000 size(64), length(11) 0000101101000000 size(64), length(10) ---------------- XOR (^) 0000011100100000 size(64), length(11) CAUTION: Unlike Vector and Hashtable, BitSet is not synchronized. You must externally synchronize access to this class when using BitSet in a multithreaded context. The collections framework has made Vector, Enumeration, Stack, Dictionary, and Hashtable obsolete. These types continue to be part of the standard class library to support legacy code. Also, the Preferences API (see Chapter 9) has made Properties largely obsolete. Because BitSet is still relevant, this class continues to be improved. CHAPTER 8: Discovering the Collections Framework 376 NOTE: It is not surprising that BitSet is being improved (as recently as Java version 6 at time of writing) when you realize the usefulness of variable-length bitsets. Because of their compactness and other advantages, variable-length bitsets are often used to implement an operating system’s priority queues and facilitate memory page allocation. Unix-oriented file systems also use bitsets to facilitate the allocation of inodes (information nodes) and disk sectors. And bitsets are useful in Huffman coding, a data-compression algorithm for achieving lossless data compression. EXERCISES The following exercises are designed to test your understanding of the collections framework: 1. What is a collection? 2. What is the collections framework? 3. The collections framework largely consists of what components? 4. What is a comparable? 5. When would you have a class implement the Comparable interface? 6. What is a comparator and what is its purpose? 7. True or false: A collection uses a comparator to define the natural ordering of its elements. 8. What does the Iterable interface describe? 9. What does the Collection interface represent? 10. Identify a situation where Collection’s add() method would throw an instance of the UnsupportedOperationException class. 11. Iterable’s iterator() method returns an instance of a class that implements the Iterator interface. What methods does this interface provide? 12. What is the purpose of the enhanced for loop statement? 13. How is the enhanced for loop statement expressed? 14. True or false: The enhanced for loop works with arrays. 15. What is autoboxing? 16. What is unboxing? 17. What is a list? CHAPTER 8: Discovering the Collections Framework 377 18. What does a ListIterator instance use to navigate through a list? 19. What is a view? 20. Why would you use the subList() method? 21. What does the ArrayList class provide? 22. What does the LinkedList class provide? 23. What is a node? 24. True or false: ArrayList provides faster element insertions and deletions than LinkedList. 25. What is a set? 26. What does the TreeSet class provide? 27. What does the HashSet class provide? 28. True or false: To avoid duplicate elements in a hashset, your own classes must correctly override equals() and hashCode(). 29. What is the difference between HashSet and LinkedHashSet? 30. What does the EnumSet class provide? 31. What is a sorted set? 32. True or false: HashSet is an example of a sorted set. 33. Why would a sorted set’s add() method throw ClassCastException when you attempt to add an element to the sorted set? 34. What is a queue? 35. True or false: Queue’s element() method throws NoSuchElementException when it is called on an empty queue. 36. What does the PriorityQueue class provide? 37. What is a map? 38. What does the TreeMap class provide? 39. What does the HashMap class provide? 40. What does a hashtable use to map keys to integer values? 41. Continuing from the previous question, what are the resulting integer values called, and what do they accomplish? 42. What is a hashtable’s capacity? 43. What is a hashtable’s load factor? CHAPTER 8: Discovering the Collections Framework 378 44. What is the difference between HashMap and LinkedHashMap? 45. What does the IdentityHashMap class provide? 46. What does the WeakHashMap class provide? 47. What does the EnumMap class provide? 48. What is a sorted map? 49. True or false: TreeMap is an example of a sorted map. 50. What is the purpose of the Arrays class’s static List asList(T... array) method? 51. True or false: Binary search is slower than linear search. 52. Which Collections method would you use to return a synchronized variation of a hashset? 53. Identify the seven legacy collections-oriented types. 54. As an example of array list usefulness, create a JavaQuiz application that presents a multiple-choice-based quiz on Java features. The JavaQuiz class’s main() method first populates the array list with the entries in the following QuizEntry array: static QuizEntry[] quizEntries = { new QuizEntry("What was Java's original name?", new String[] { "Oak", "Duke", "J", "None of the above" }, 'A'), new QuizEntry("Which of the following reserved words is also a literal?", new String[] { "for", "long", "true", "enum" }, 'C'), new QuizEntry("The conditional operator (?:) resembles which statement?", new String[] { "switch", "if-else", "if", "while" }, 'B') }; Each QuizEntry instance consists of a question, four possible answers, and the letter (A, B, C, or D) of the correct answer. main() then uses the array list’s iterator() method to return an Iterator instance, and this instance’s hasNext() and next() methods to iterate over the list. Each of the iterations outputs the question and four possible answers, and then prompts the user to enter the correct choice. After the user enters A, B, C, or D, main() outputs a message stating whether or not the user made the correct choice. 55. Why is (int) (f^(f>>>32)) used instead of (int) (f^(f>>32)) in the hash code generation algorithm? CHAPTER 8: Discovering the Collections Framework 379 56. Collections provides the static int frequency(Collection collection, Object o) method to return the number of collection elements that are equal to o. Create a FrequencyDemo application that reads its command-line arguments and stores all arguments except for the last argument in a list, and then calls frequency() with the list and last command-line argument as this method’s arguments. It then outputs this method’s return value (the number of occurrences of the last command-line argument in the previous command-line arguments). For example, java FrequencyDemo should output Number of occurrences of null = 0, and java FrequencyDemo how much wood could a woodchuck chuck if a woodchuck could chuck wood wood should output Number of occurrences of wood = 2. Summary A collection is a group of objects that are stored in an instance of a class designed for this purpose. To save you from having to create your own collections classes, Java provides the standardized collections framework for representing and manipulating collections. The collections framework largely consists of core interfaces, implementation classes, and the Arrays and Collections utility classes. The core interfaces make it possible to manipulate collections independently of their implementations. The collections framework’s core interfaces include Iterable, Collection, List, Set, SortedSet, Queue, Map, and SortedMap. Collection extends Iterable; List, Set, and Queue each extend Collection; SortedSet extends Set; and SortedMap extends Map. The framework’s implementation classes include the concrete ArrayList, LinkedList, TreeSet, HashSet, LinkedHashSet, EnumSet, PriorityQueue, TreeMap, HashMap, LinkedHashMap, IdentityHashMap, WeakHashMap, and EnumMap classes. The framework’s implementation classes also include the abstract AbstractCollection, AbstractList, AbstractSequentialList, AbstractSet, AbstractQueue, and AbstractMap classes, which offer skeletal implementations of the core interfaces. The collections framework would not be complete without its Arrays and Collections utility classes. Each class supplies various utility (static) methods that implement useful algorithms in the contexts of arrays and collections. Prior to Java version 1.2’s introduction of the collections framework, developers could create their own frameworks, or use the Vector, Enumeration, Stack, Dictionary, Hashtable, Properties, and BitSet types, which were introduced by Java version 1.0. 8CHAPTER 8: Discovering the Collections Framework 380 The collections framework has made Vector, Enumeration, Stack, Dictionary, and Hashtable obsolete. Also, the Preferences API (see Chapter 9) has made Properties largely obsolete. Because BitSet is still relevant, this class continues to be improved. Your exploration of Java’s utility APIs is far from finished. Chapter 9 continues to focus on utility APIs by discussing the concurrency utilities, the internationalization APIs, the Preferences API, Random, and the Regular Expressions API. 381 381 Chapter Discovering Additional Utility APIs Chapter 9 continues to explore Java’s utility APIs by introducing APIs that help you work with threads in an easier way, internationalize your applications to reach a wider audience, save configuration preferences in persistent storage, obtain random numbers in a more flexible manner, and quickly search and parse strings. NOTE: Chapter 9 explores utility API classes and interfaces that are located in the java.text and java.util packages, and java.util’s concurrent, prefs, and regex subpackages. Concurrency Utilities Java version 5 introduced the concurrency utilities, classes and interfaces that simplify the development of concurrent (multithreaded) applications. These types are located in the java.util.concurrent package and in its java.util.concurrent.atomic and java.util.concurrent.locks subpackages. The concurrency utilities leverage the low-level Threading API (see Chapter 7) in their implementations and provide higher-level building blocks (such as locking idioms) to make it easier to create multithreaded applications. These utilities are organized into executor, synchronizer, concurrent collection, lock, and atomic variable categories. Executors Chapter 7 introduced the Threading API, which lets you execute runnable tasks via expressions such as new Thread(new RunnableTask()).start();. These expressions tightly couple task submission with the task’s execution mechanics (run on the current thread, a new thread, or a thread arbitrarily chosen from a pool [group] of threads). 9 CHAPTER 9: Discovering Additional Utility APIs 382 NOTE: A task is an object whose class implements the java.lang.Runnable interface (a runnable task) or the java.util.concurrent.Callable interface (a callable task). The concurrency utilities provide executors as a high-level alternative to low-level Threading API expressions for executing runnable tasks. An executor is an object whose class directly or indirectly implements the java.util.concurrent.Executor interface, which decouples task submission from task-execution mechanics. NOTE: The executor framework’s use of interfaces to decouple task submission from task- execution mechanics is analogous to the collections framework’s use of core interfaces to decouple lists, sets, queues, and maps from their implementations. Decoupling results in flexible code that is easier to maintain. Executor declares a solitary void execute(Runnable runnable) method that executes the runnable task named runnable at some point in the future. execute() throws java.lang.NullPointerException when runnable is null, and java.util.concurrent.RejectedExecutionException when it cannot execute runnable. NOTE: RejectedExecutionException can be thrown when an executor is shutting down and does not want to accept new tasks. Also, this exception can be thrown when the executor does not have enough room to store the task (perhaps the executor uses a bounded blocking queue to store tasks and the queue is full—I discuss blocking queues later in this chapter). Listing 9–1 presents the Executor equivalent of the aforementioned new Thread(new RunnableTask()).start(); expression. Listing 9–1. Decoupling runnable task submission from task-execution mechanics Executor executor = ...; // ... represents some executor creation executor.execute(new RunnableTask()); Although Executor is easy to use, this interface is limited in various ways:  Executor focuses exclusively on Runnable. Because Runnable’s run() method does not return a value, there is no convenient way for a runnable task to return a value to its caller.  Executor does not provide a way to track the progress of executing runnable tasks, cancel an executing runnable task, or determine when the runnable task finishes execution.  Executor cannot execute a collection of runnable tasks.  Executor does not provide a way for an application to shut down an executor (much less to properly shut down an executor). CHAPTER 9: Discovering Additional Utility APIs 383 These limitations are addressed by the java.util.concurrent.ExecutorService interface, which extends Executor, and whose implementation is typically a thread pool. Table 9–1 describes ExecutorService’s methods. Table 9–1. ExecutorService Methods Method Description boolean awaitTermination (long timeout, TimeUnit unit) Block (wait) until all tasks have finished after a shutdown request, the timeout (measured in unit time units) expires, or the current thread is interrupted, whichever happens first. Return true when this executor has terminated, and false when the timeout elapses before termination. This method throws java.lang.InterruptedException when interrupted. List> invokeAll(Collection> tasks) Execute each callable task in the tasks collection, and return a List of Future instances that hold task statuses and results when all tasks complete—a task completes through normal termination or by throwing an exception. The List of Futures is in the same sequential order as the sequence of tasks returned by tasks’ iterator. This method throws InterruptedException when it is interrupted while waiting, in which case unfinished tasks are canceled, NullPointerException when tasks or any of its elements is null, and RejectedExecutionException when any one of tasks’ tasks cannot be scheduled for execution. List> invokeAll(Collection> tasks, long timeout, TimeUnit unit) Execute each callable task in the tasks collection, and return a List of Future instances that hold task statuses and results when all tasks complete—a task completes through normal termination or by throwing an exception—or the timeout (measured in unit time units) expires. Tasks that are not completed at expiry are canceled. The List of Futures is in the same sequential order as the sequence of tasks returned by tasks’ iterator. This method throws InterruptedException when it is interrupted while waiting, in which case unfinished tasks are canceled, NullPointerException when tasks or any of its elements is null, and RejectedExecutionException when any one of tasks’ tasks cannot be scheduled for execution. T invokeAny (Collection> tasks) Execute the given tasks, returning the result of an arbitrary task that has completed successfully (i.e., without throwing an exception), if any does. Upon normal or exceptional return, tasks that have not completed are canceled. This method throws InterruptedException when it is interrupted while waiting, NullPointerException when tasks or any of its elements is null, java.lang.IllegalArgumentException when tasks is empty, java.util.concurrent.ExecutionException when no task completes successfully, and RejectedExecutionException when none of the tasks can be scheduled for execution. CHAPTER 9: Discovering Additional Utility APIs 384 Method Description T invokeAny (Collection> tasks, long timeout, TimeUnit unit) Execute the given tasks, returning the result of an arbitrary task that has completed successfully (i.e., without throwing an exception), if any does before the timeout (measured in unit time units) expires— tasks that are not completed at expiry are canceled. Upon normal or exceptional return, tasks that have not completed are canceled. This method throws InterruptedException when it is interrupted while waiting, NullPointerException when tasks or any of its elements is null, IllegalArgumentException when tasks is empty, java.util.concurrent.TimeoutException when the timeout elapses before any task successfully completes, ExecutionException when no task completes successfully, and RejectedExecutionException when none of the tasks can be scheduled for execution. boolean isShutdown() Return true when this executor has been shut down; otherwise, return false. boolean isTerminated() Return true when all tasks have completed following shutdown; otherwise, return false. This method will never return true prior to shutdown() or shutdownNow() being called. void shutdown() Initiate an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. Calling this method has no effect after the executor has shut down. List shutdownNow() Attempt to stop all actively executing tasks, halt the processing of waiting tasks, and return a list of the tasks that were awaiting execution. There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via Thread.interrupt(), so any task that fails to respond to interrupts may never terminate. Future submit (Callable task) Submit a callable task for execution and return a Future instance representing task’s pending results. The Future instance’s get() method returns task’s result upon successful completion. This method throws RejectedExecutionException when task cannot be scheduled for execution, and NullPointerException when task is null. Future submit (Runnable task) Submit a runnable task for execution and return a Future instance whose get() method returns the null reference upon successful completion. This method throws RejectedExecutionException when task cannot be scheduled for execution, and NullPointerException when task is null. Future submit (Runnable task, T result) Submit a runnable task for execution and return a Future instance whose get() method returns result upon successful completion. This method throws RejectedExecutionException when task cannot be scheduled for execution, and NullPointerException when task is null. This book was purchased by harshamv@variable3.comCHAPTER 9: Discovering Additional Utility APIs 385 Table 9–1 refers to java.util.concurrent.TimeUnit, an enum that represents time durations at given units of granularity: DAYS, HOURS, MICROSECONDS, MILLISECONDS, MINUTES, NANOSECONDS, and SECONDS. Furthermore, TimeUnit declares methods for converting across units, and for performing timing and delay operations in these units. Table 9–1 also refers to callable tasks, which are analogous to runnable tasks. Unlike Runnable, whose void run() method cannot throw checked exceptions, Callable declares a V call() method that returns a value, and which can throw checked exceptions because call() is declared with a throws Exception clause. Finally, Table 9–1 refers to the java.util.concurrent.Future interface, which represents the result of an asynchronous computation. Future, whose generic type is Future, provides methods for canceling a task, for returning a task’s value, and for determining whether or not the task has finished. Table 9–2 describes Future’s methods. Table 9–2. Future Methods Method Description boolean cancel(boolean mayInterruptIfRunning) Attempt to cancel execution of this task, and return true when the task was cancelled; otherwise, return false (perhaps the task completed normally before this method was called). The cancellation attempt fails when the task has completed, has already been cancelled, or could not be cancelled for some other reason. If successful and this task had not started when cancel() was called, the task should never run. If the task has already started, then mayInterruptIfRunning determines whether the thread executing this task should be interrupted (true) or not (false) in an attempt to stop the task. After this method returns, subsequent calls to isDone() always return true. Subsequent calls to isCancelled() always return true when cancel() returns true. V get() Wait if necessary for the task to complete and then return the result. This method throws java.util.concurrent.CancellationException when the task was cancelled prior to this method being called, ExecutionException when the task threw an exception, and InterruptedException when the current thread was interrupted while waiting. V get(long timeout, TimeUnit unit) Wait at most timeout units (as specified by unit) for the task to complete and then return the result. This method throws CancellationException when the task was cancelled prior to this method being called, ExecutionException when the task threw an exception, InterruptedException when the current thread was interrupted while waiting, and TimeoutException when this method’s timeout value expires (the wait times out). boolean isCancelled() Return true when this task was cancelled before it completed normally. boolean isDone() Return true when this task completed. Completion may be due to normal termination, an exception, or cancellation—this method returns true in all of these cases. CHAPTER 9: Discovering Additional Utility APIs 386 Suppose you intend to write an application whose graphical user interface lets the user enter a word. Once the user enters the word, the application presents this word to several online dictionaries and obtains each dictionary’s entry. These entries are subsequently displayed to the user. Because online access can be slow, and because the user interface should remain responsive (perhaps the user might want to end the application), you offload the “obtain word entries” task to an executor that runs this task on a separate thread. Listing 9–2 employs ExecutorService, Callable, and Future to accomplish this objective. Listing 9–2. An executor-based skeletal framework for obtaining word entries from online dictionaries ExecutorService executor = ...; // ... represents some executor creation Future taskFuture = executor.submit(new Callable() { public String[] call() { String[] entries = ...; // Access online dictionaries // with search word and populate // entries with their resulting // entries. return entries; } }); // Do stuff. String entries = taskFuture.get(); After obtaining an executor in some manner (you will learn how shortly), Listing 9–2’s main thread submits a callable task to the executor. The submit() method immediately returns with a reference to a Future object for controlling task execution and accessing results. The main thread ultimately calls this object’s get() method to get these results. NOTE: The java.util.concurrent.ScheduledExecutorService interface extends ExecutorService and describes an executor that lets you schedule tasks to run once or to execute periodically after a given delay. Although you could create your own Executor, ExecutorService, and ScheduledExecutorService implementations (such as class DirectExecutor implements Executor { public void execute(Runnable r) { r.run(); } }—run executor directly on the calling thread), the concurrency utilities offer a simpler alternative: Executors. TIP: If you intend to create your own ExecutorService implementations, you will find it helpful to work with the java.util.concurrent.AbstractExecutorService and java.util.concurrent.FutureTask classes. CHAPTER 9: Discovering Additional Utility APIs 387 The java.util.concurrent.Executors utility class declares several static methods that return instances of various ExecutorService and ScheduledExecutorService implementations (and other kinds of instances). This class’s utility methods accomplish the following tasks:  Create and return an ExecutorService instance that is configured with commonly used configuration settings.  Create and return a ScheduledExecutorService instance that is configured with commonly used configuration settings.  Create and return a “wrapped” ExecutorService or ScheduledExecutorService instance, and disable reconfiguration of the executor service by making implementation-specific methods inaccessible.  Create and return a java.util.concurrent.ThreadFactory instance for creating new threads.  Create and return a Callable instance out of other closure-like forms (such as a Runnable instance) so that they can be used in execution methods (such as ExecutorService’s submit(Callable) method) that require Callable arguments. For example, static ExecutorService newFixedThreadPool(int nThreads) creates a thread pool that reuses a fixed number of threads operating off of a shared unbounded queue. At most, nThreads threads are actively processing tasks. If additional tasks are submitted when all threads are active, they wait in the queue for an available thread. If any thread terminates because of a failure during execution before the executor shuts down, a new thread will take its place when needed to execute subsequent tasks. The threads in the pool will exist until the executor is explicitly shut down. This method throws IllegalArgumentException when you pass zero or a negative value to nThreads. NOTE: Thread pools are used to eliminate the overhead from having to create a new thread for each submitted task. Thread creation is not cheap, and having to create many threads could severely impact an application’s performance. You would commonly use executors, runnables, callables, and futures in an input/output context. (I discuss input/output in Chapter 10.) Performing a lengthy calculation offers another scenario where you could use these types. For example, Listing 9–3 uses an executor, a callable, and a future in a calculation context of Euler’s number e (2.71828…). Listing 9–3. Calculating Euler’s number e import java.math.BigDecimal; import java.math.MathContext; import java.math.RoundingMode; import java.util.concurrent.Callable; CHAPTER 9: Discovering Additional Utility APIs 388 import java.util.concurrent.ExecutionException; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Future; public class CalculateE { final static int LASTITER = 17; public static void main(String[] args) { ExecutorService executor = Executors.newFixedThreadPool(1); Callable callable; callable = new Callable() { public BigDecimal call() { MathContext mc = new MathContext(100, RoundingMode.HALF_UP); BigDecimal result = BigDecimal.ZERO; for (int i = 0; i <= LASTITER; i++) { BigDecimal factorial = factorial(new BigDecimal (i)); BigDecimal res = BigDecimal.ONE.divide(factorial, mc); result = result.add(res); } return result; } public BigDecimal factorial(BigDecimal n) { if (n.equals(BigDecimal.ZERO)) return BigDecimal.ONE; else return n.multiply(factorial(n.subtract(BigDecimal.ONE))); } }; Future taskFuture = executor.submit(callable); try { while (!taskFuture.isDone()) System.out.println("waiting"); System.out.println(taskFuture.get()); } catch(ExecutionException ee) { System.err.println("task threw an exception"); System.err.println(ee); } catch(InterruptedException ie) { System.err.println("interrupted while waiting"); } executor.shutdownNow(); } } CHAPTER 9: Discovering Additional Utility APIs 389 The main thread first obtains an executor by calling Executors’ newFixedThreadPool() method. It then instantiates an anonymous class that implements Callable and submits this task to the executor, receiving a Future instance in response. After submitting a task, a thread typically does some other work until it needs to obtain the task’s result. I have chosen to simulate this work by having the main thread repeatedly output a waiting message until the Future instance’s isDone() method returns true. (In a realistic application, I would avoid this looping.) At this point, the main thread calls the instance’s get() method to obtain the result, which is then output. CAUTION: It is important to shut down the executor after it completes; otherwise, the application might not end. The executor accomplishes this task by calling shutdownNow(). The callable’s call() method calculates e by evaluating the mathematical power series e = 1/0!+1/1!+1/2!+…. This series can be evaluated by summing 1/n!, where n ranges from 0 to infinity. call() first instantiates java.math.MathContext to encapsulate a precision (number of digits) and a rounding mode. I chose 100 as an upper limit on e’s precision, and I also chose HALF_UP as the rounding mode. TIP: Increase the precision as well as MAXITER’s value to converge the series to a lengthier and more accurate approximation of e. call() next initializes a BigDecimal local variable named result to BigDecimal.ZERO. It then enters a loop that calculates a factorial, divides BigDecimal.ONE by the factorial, and adds the division result to result. The divide() method takes the MathContext instance as its second argument to ensure that the division does not result in a nonterminating decimal expansion, which throws java.lang.ArithmeticException, which the executor rethrows as ExecutionException. When you run this application, you should observe output similar to the following: waiting waiting waiting waiting waiting waiting waiting 2.718281828459045070516047795848605061178979635251032698900735004065225042504843314055 887974344245741730039454062711 CHAPTER 9: Discovering Additional Utility APIs 390 Synchronizers The Threading API offers synchronization primitives for synchronizing thread access to critical sections. Because it can be difficult to correctly write synchronized code that is based on these primitives, the concurrency utilities include synchronizers, classes that facilitate common forms of synchronization. Four commonly used synchronizers are countdown latches, cyclic barriers, exchangers, and semaphores:  A countdown latch lets one or more threads wait at a “gate” until another thread opens this gate, at which point these other threads can continue. The java.util.concurrent.CountDownLatch class implements this synchronizer.  A cyclic barrier lets a group of threads wait for each other to reach a common barrier point. The java.util.concurrent.CyclicBarrier class implements this synchronizer, and makes use of the java.util.concurrent.BrokenBarrierException class.  An exchanger lets a pair of threads exchange objects at a synchronization point. The java.util.concurrent.Exchanger class implements this synchronizer.  A semaphore maintains a set of permits for restricting the number of threads that can access a limited resource. The java.util.concurrent.Semaphore class implements this synchronizer. Consider the CountDownLatch class. Each of its instances is initialized to a nonzero count. A thread calls one of CountDownLatch’s await() methods to block until the count reaches zero. Another thread calls CountDownLatch’s countDown() method to decrement the count. Once the count reaches zero, the waiting threads are allowed to continue. NOTE: Once waiting threads are released, subsequent calls to await() return immediately. Also, because the count cannot be reset, a CountDownLatch instance can be used only once. When repeated use is a requirement, use the CyclicBarrier class instead. We can use CountDownLatch to ensure that threads start working at approximately the same time. For example, check out Listing 9–4. Listing 9–4. Using a countdown latch to trigger a coordinated start import java.util.concurrent.CountDownLatch; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class CountDownLatchDemo { final static int NTHREADS = 3; public static void main(String[] args) CHAPTER 9: Discovering Additional Utility APIs 391 { final CountDownLatch enterLatch = new CountDownLatch(NTHREADS); Runnable r = new Runnable() { public void run() { try { report("entered run()"); enterLatch.countDown(); enterLatch.await(); report("doing work"); } catch (InterruptedException ie) { System.err.println(ie); } } void report(String s) { System.out.println(System.currentTimeMillis() + ": " + Thread.currentThread() + ": " + s); } }; ExecutorService executor = Executors.newFixedThreadPool(NTHREADS); for (int i = 0; i < NTHREADS; i++) executor.execute(r); try { Thread.sleep(3000); // Sleep for 3 seconds. } catch (InterruptedException ie) { System.err.println(ie); } executor.shutdownNow(); } } Listing 9–4’s main thread first creates a countdown latch with a count initialized to the number of threads that must start working at approximately the same time. It then creates a runnable whose run() method is executed by subsequently created threads. The runnable’s run() method first outputs an initial message and then calls the latch instance’s countDown() method to decrement the count. It next calls await() to wait for the latch’s count to reach zero, at which time it can output its work message. After obtaining an executor that is based on a thread pool of NTHREADS threads, the main thread calls the executor’s execute() method NTHREADS times, passing the runnable to each of the NTHREADS pool-based threads. The main thread then sleeps for three seconds to give these threads a chance to demonstrate the countdown latch before the main thread shuts down the executor by calling shutdownNow(). When you run this application, you will observe output similar to the following: CHAPTER 9: Discovering Additional Utility APIs 392 1279656117609: Thread[pool-1-thread-1,5,main]: entered run() 1279656117609: Thread[pool-1-thread-2,5,main]: entered run() 1279656117609: Thread[pool-1-thread-3,5,main]: entered run() 1279656117609: Thread[pool-1-thread-1,5,main]: doing work 1279656117609: Thread[pool-1-thread-2,5,main]: doing work 1279656117609: Thread[pool-1-thread-3,5,main]: doing work NOTE: For brevity, I have avoided examples that demonstrate CyclicBarrier, Exchanger, and Semaphore. Instead, I refer you to the Java documentation for these classes. Each class’s documentation provides an example that shows you how to use the class. Concurrent Collections The java.util.concurrent package includes several interfaces and classes that are concurrency-oriented extensions to the collections framework (see Chapter 8):  BlockingQueue is a subinterface of java.util.Queue that describes a first-in, first-out (FIFO) data structure. It provides additional operations that wait for the queue to become nonempty before retrieving an element, and wait for space to become available in the queue before storing an element. Each of the ArrayBlockingQueue, LinkedBlockingQueue, PriorityBlockingQueue, and SynchronousQueue classes implements this interface.  ConcurrentMap is a subinterface of java.util.Map that declares additional atomic putIfAbsent(), remove(), and replace() methods. The ConcurrentHashMap class (the concurrent equivalent of java.util.HashMap) implements this interface.  ConcurrentLinkedQueue is an unbounded thread-safe FIFO implementation of the Queue interface. Listing 9–5 uses BlockingQueue and ArrayBlockingQueue in an alternative to Listing 7- 13’s producer-consumer application (PC). Listing 9–5. The blocking queue equivalent of Listing 7-13’s PC application import java.util.concurrent.ArrayBlockingQueue; import java.util.concurrent.BlockingQueue; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class PC { public static void main(String[] args) { final BlockingQueue bq; bq = new ArrayBlockingQueue(26); final ExecutorService executor = Executors.newFixedThreadPool(2); Runnable producer; producer = new Runnable() CHAPTER 9: Discovering Additional Utility APIs 393 { public void run() { for (char ch = 'A'; ch <= 'Z'; ch++) { try { bq.put(ch); System.out.println(ch + " produced by producer."); } catch (InterruptedException ie) { assert false; } } } }; executor.execute(producer); Runnable consumer; consumer = new Runnable() { public void run() { char ch = '\0'; do { try { ch = bq.take(); System.out.println(ch + " consumed by consumer."); } catch (InterruptedException ie) { assert false; } } while (ch != 'Z'); executor.shutdownNow(); } }; executor.execute(consumer); } } Listing 9–5 uses BlockingQueue’s put() and take() methods, respectively, to put an object on the blocking queue and to remove an object from the blocking queue. put() blocks when there is no room to put an object; take() blocks when the queue is empty. Although BlockingQueue ensures that a character is never consumed before it is produced, this application’s output may indicate otherwise. For example, here is a portion of the output from one run: Y consumed by consumer. Y produced by producer. Z consumed by consumer. Z produced by producer. CHAPTER 9: Discovering Additional Utility APIs 394 Chapter 7’s PC application overcame this incorrect output order by introducing an extra layer of synchronization around setSharedChar()/System.out.println() and an extra layer of synchronization around getSharedChar()/System.out.println(). The next section shows you an alternative in the form of locks. Locks The java.util.concurrent.locks package provides interfaces and classes for locking and waiting for conditions in a manner that is distinct from built-in synchronization and monitors. This package’s most basic lock interface is Lock, which provides more extensive locking operations than can be achieved via the synchronized reserved word. Lock also supports a wait/notification mechanism through associated Condition objects. NOTE: The biggest advantage of Lock objects over the implicit locks that are obtained when threads enter critical sections (controlled via the synchronized reserved word) is their ability to back out of an attempt to acquire a lock. For example, the tryLock() method backs out if the lock is not available immediately or before a timeout expires (if specified). Also, the lockInterruptibly() method backs out when another thread sends an interrupt before the lock is acquired. ReentrantLock implements Lock, describing a reentrant mutual exclusion Lock implementation with the same basic behavior and semantics as the implicit monitor lock accessed via synchronized, but with extended capabilities. Listing 9–6 demonstrates Lock and ReentrantLock in a version of Listing 9–5 that ensures that the output is never shown in incorrect order (a consumed message appearing before a produced message). Listing 9–6. Achieving synchronization in terms of locks import java.util.concurrent.ArrayBlockingQueue; import java.util.concurrent.BlockingQueue; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; public class PC { public static void main(String[] args) { final Lock lock = new ReentrantLock(); final BlockingQueue bq; bq = new ArrayBlockingQueue(26); final ExecutorService executor = Executors.newFixedThreadPool(2); Runnable producer; producer = new Runnable() This book was purchased by harshamv@variable3.comCHAPTER 9: Discovering Additional Utility APIs 395 { public void run() { for (char ch = 'A'; ch <= 'Z'; ch++) { try { lock.lock(); try { while (!bq.offer(ch)) { lock.unlock(); Thread.sleep(50); lock.lock(); } System.out.println(ch + " produced by producer."); } catch (InterruptedException ie) { assert false; } } finally { lock.unlock(); } } } }; executor.execute(producer); Runnable consumer; consumer = new Runnable() { public void run() { char ch = '\0'; do { try { lock.lock(); try { Character c; while ((c = bq.poll()) == null) { lock.unlock(); Thread.sleep(50); lock.lock(); } ch = c; // unboxing behind the scenes System.out.println(ch + " consumed by consumer."); } catch (InterruptedException ie) { assert false; CHAPTER 9: Discovering Additional Utility APIs 396 } } finally { lock.unlock(); } } while (ch != 'Z'); executor.shutdownNow(); } }; executor.execute(consumer); } } Listing 9–6 uses Lock’s lock() and unlock() methods to obtain and release a lock. When a thread calls lock() and the lock is unavailable, the thread is disabled (and cannot be scheduled) until the lock becomes available. This listing also uses BlockingQueue’s offer() method instead of put() to store an object in the blocking queue, and its poll() method instead of take() to retrieve an object from the queue. These alternative methods are used because they do not block. If I had used put() and take(), this application would have deadlocked in the following scenario: 1. The consumer thread acquires the lock via its lock.lock() call. 2. The producer thread attempts to acquire the lock via its lock.lock() call and is disabled because the consumer thread has already acquired the lock. 3. The consumer thread calls take() to obtain the next Character object from the queue. 4. Because the queue is empty, the consumer thread must wait. 5. The consumer thread does not give up the lock that the producer thread requires before waiting, so the producer thread also continues to wait. NOTE: If I had access to the private lock used by BlockingQueue implementations, I would have used put() and take(), and also would have called Lock’s lock() and unlock() methods on that lock. The resulting application would then have been identical (from a lock perspective) to Listing 7-13’s PC application, which used synchronized twice for each of the producer and consumer threads. Run this application and you will discover that it generates the same output as Listing 7- 13’s PC application. CHAPTER 9: Discovering Additional Utility APIs 397 Atomic Variables The java.util.concurrent.atomic package provides Atomic-prefixed classes (such as AtomicLong) that support lock-free, thread-safe operations on single variables. Each class declares methods such as get() and set() to read and write this variable without the need for external synchronization. Listing 7-9 declared a small utility class named ID for returning unique long integer identifiers via ID’s getNextID() method. Because this method was not synchronized, multiple threads could obtain the same identifier. Listing 9–7 fixes this problem by including reserved word synchronized in the method header. Listing 9–7. Returning unique identifiers in a thread-safe manner via synchronized public class ID { private static long nextID = 0; public static synchronized long getNextID() { return nextID++; } } Although synchronized is appropriate for this class, excessive use of this reserved word in more complex classes can lead to deadlock, starvation, or other problems. Listing 9–8 shows you how to avoid these assaults on a concurrent application’s liveness (the ability to execute in a timely manner) by replacing synchronized with an atomic variable. Listing 9–8. Returning unique IDs in a thread-safe manner via AtomicLong public class ID { private static AtomicLong nextID = new AtomicLong(0); public static long getNextID() { return nextID.getAndIncrement(); } } In Listing 9–8, I have converted nextID from a long to an AtomicLong instance, initializing this object to 0. I have also refactored the getNextID() method to call AtomicLong’s getAndIncrement() method, which increments the AtomicLong instance’s internal long integer variable by 1 and returns the previous value in one indivisible step. Internationalization APIs We tend to write software that reflects our cultural backgrounds. For example, a Spanish developer’s application might present Spanish text, an Arabic developer’s application might present a Hijri (Islamic) calendar, and a Japanese developer’s application might display its currencies using the Japanese Yen currency symbol. Because cultural biases restrict the size of an application’s audience, you might consider internationalizing your Android and Java SE applications to reach a larger audience (and make more money). CHAPTER 9: Discovering Additional Utility APIs 398 Internationalization is the process of creating an application that automatically adapts to its current user’s culture so that the user can read the application’s text, hear audio clips in the user’s language (if audio is supported), and so on. Java simplifies internationalization by supporting Unicode (a universal character set that encodes the various symbols making up the world’s written languages) via char (see Chapter 2) and java.lang.Character (see Chapter 6), and by offering the APIs discussed in this section. Locales The java.util.Locale class is the centerpiece of the various internationalization APIs. Instances of this class represent locales, which are geographical, political, or cultural regions. NOTE: Java version 6 introduced the Locale-Sensitive Services SPI (Service Provider Interface) to let third parties support unsupported locales by introducing new implementations of various locale-sensitive classes located in the java.text and java.util packages. For example, you can use this SPI to provide new implementations of the java.util.BreakIterator class (discussed later in this chapter). To learn about Locale-Sensitive Services, and to explore an example that introduces a new currency, check out this topic in my book Beginning Java SE 6 Platform: From Novice to Professional (Apress, 2007; ISBN: 159059830). Locale declares constants (such as CANADA) that describe some common locales. This class also declares three constructors for initializing Locale objects, in case you cannot find an appropriate Locale constant for a specific locale:  Locale(String language) initializes a Locale instance to a language code; for example, "fr" for French.  Locale(String language, String country) initializes a Locale instance to a language code and a country code; for example, "en" for English and "US" for United States.  Locale(String language, String country, String variant) initializes a Locale instance to a language code, a country code, and a vendor- or browser-specific variant code; for example, "de" for German, "DE" for Germany, and "WIN" for Windows (or "MAC" for Macintosh). The International Standards Organization (ISO) defines language and country codes. ISO 639 (http://ftp.ics.uci.edu/pub/ietf/http/related/iso639.txt) defines language codes. ISO 3166 (http://userpage.chemie.fu-berlin.de/diverse/doc/ISO_3166.html) defines country codes. Locale works with both standards. CHAPTER 9: Discovering Additional Utility APIs 399 NOTE: ISO 639 is not a stable standard and some language codes have changed; specifically, iw, ji, and in have changed to he, yi, and id, respectively. Variant codes are useful for dealing with computing platform differences. For example, font differences may force you to use different characters on Windows-, Linux-, and Unix-based operating systems (such as Solaris). Unlike language and country codes, variant codes are not standardized. Although applications can create their own Locale objects (perhaps to let users choose from similar locales), they will often call API methods that work with the default locale, which is the locale made available to the virtual machine at startup. An application can call Locale’s static Locale getDefault() method if it needs to access this locale. For testing or other purposes, the application can override the default locale by calling Locale’s static void setDefault(Locale locale) method. setDefault() sets the default locale to locale. However, passing null to locale causes setDefault() to throw NullPointerException. Listing 9–9 demonstrates getDefault() and setDefault(). Listing 9–9. Viewing and changing the default locale import java.util.Locale; public class MyLocale { public static void main(String[] args) { System.out.println(Locale.getDefault()); Locale.setDefault(Locale.CANADA); System.out.println(Locale.getDefault()); } } When I run this application, I observe the following output: en_US en_CA You can change the default locale that is made available to the virtual machine by assigning appropriate values to the user.language and user.country system properties when you launch the application via the java tool. For example, the following java command line changes the default locale to fr_FR: java -Duser.language=fr -Duser.country=FR MyLocale As you continue to explore Locale, you will discover additional useful methods. For example, static String[] getISOLanguages() returns an array of ISO 639 language codes (including former and changed codes), and static String[] getISOCountries() returns an array of ISO 3166 country codes. CHAPTER 9: Discovering Additional Utility APIs 400 NOTE: Read John O’Conner’s “Internationalization: Understanding Locale in the Java Platform” article (http://java.sun.com/developer/technicalArticles/J2SE/locale/) for more information on Locale. Resource Bundles An internationalized application contains no hard-coded text or other locale-specific elements (such as a specific currency format). Instead, each supported locale’s version of these elements is stored outside of the application. NOTE: Creating a set of locale-specific elements is known as localization. Java is responsible for storing each locale’s version of certain elements, such as currency formats. In contrast, it is your responsibility to store each supported locale’s version of other elements, such as text, audio clips, and locale-sensitive images. Java facilitates this element storage by providing resource bundles, which are containers that hold one or more locale-specific elements, and which are each associated with one and only one locale. Many applications work with one or more resource bundle families. Each family consists of resource bundles for all supported locales and typically contains one kind of element (perhaps text, or audio clips that contain language-specific verbal instructions). Each family also shares a common family name (also known as a base name); each of its resource bundles has a unique locale designation that is appended to the family name, to differentiate one resource bundle from another within the family. Consider an internationalized text-based game application for English and French users. After choosing game as the family name, and en and fr as the English and French locale designations, you end up with the following complete resource bundle names:  game_en is the complete resource bundle name for English users.  game_fr is the complete resource bundle name for French users. Although you can store all of your game’s English text in the game_en resource bundle, you might want to differentiate between American and British text (such as elevator versus lift). This differentiation leads to the following complete resource bundle names:  game_en_US is the complete resource bundle name for users who speak the United States version of the English language.  game_en_GB is the complete resource bundle name for users who speak the British version of the English language. CHAPTER 9: Discovering Additional Utility APIs 401 An application loads its resource bundles by calling the various getBundle() utility methods that are located in the abstract java.util.ResourceBundle class. For example, the application might call the following getBundle() factory methods:  static ResourceBundle getBundle(String baseName) loads a resource bundle using the specified baseName and the default locale. For example, ResourceBundle resources = ResourceBundle.getBundle("game"); attempts to load the resource bundle whose base name is game, and whose locale designation matches the default locale. If the default locale is en_US, getBundle() attempts to load game_en_US.  static ResourceBundle getBundle(String baseName, Locale locale) loads a resource bundle using the specified baseName and locale. For example, ResourceBundle resources = ResourceBundle.getBundle("game", new Locale("zh", "CN", "WIN")); attempts to load the resource bundle whose base name is game, and whose locale designation is Chinese with a Windows variant. In other words, getBundle() attempts to load game_zh_CN_WIN. NOTE: ResourceBundle is an example of a pattern that you will discover throughout the internationalization APIs. With few exceptions, each API is architected around an abstract entry- point class whose factory methods return instances of concrete subclasses. If the resource bundle identified by the base name and locale designation does not exist, the getBundle() methods search for the next closest bundle. For example, if the locale is en_US and game_en_US does not exist, getBundle() looks for game_en. The getBundle() methods first generate a sequence of candidate bundle names for the specified locale (language1, country1, and variant1) and the default locale (language2, country2, and variant2) in the following order:  baseName + "_" + language1 + "_" + country1 + "_" + variant1  baseName + "_" + language1 + "_" + country1  baseName + "_" + language1  baseName + "_" + language2 + "_" + country2 + "_" + variant2  baseName + "_" + language2 + "_" + country2  baseName + "_" + language2  baseName Candidate bundle names in which the final component is an empty string are omitted from the sequence. For example, if country1 is an empty string, the second candidate bundle name is omitted. CHAPTER 9: Discovering Additional Utility APIs 402 The getBundle() methods iterate over the candidate bundle names to find the first name for which they can instantiate an actual resource bundle. For each candidate bundle name, getBundle() attempts to create a resource bundle as follows:  It first attempts to load a class that extends the abstract java.util.ListResourceBundle class using the candidate bundle name. If such a class can be found and loaded using the specified class loader, is assignment compatible with ResourceBundle, is accessible from ResourceBundle, and can be instantiated, getBundle() creates a new instance of this class and uses it as the result resource bundle.  Otherwise, getBundle() attempts to locate a properties file. It generates a pathname from the candidate bundle name by replacing all “.” characters with “/” and appending “.properties.” It attempts to find a “resource” with this name using java.lang.ClassLoader. getResource(). (Note that a “resource” in the sense of getResource() has nothing to do with the contents of a resource bundle; it is just a container of data, such as a file.) If getResource() finds a “resource,” it attempts to create a new java.util.PropertyResourceBundle instance from its contents. If successful, this instance becomes the result resource bundle. If no result resource bundle is found, getBundle() throws an instance of the java.util.MissingResourceException class; otherwise, getBundle() instantiates the bundle’s parent resource bundle chain. NOTE: The parent resource bundle chain makes it possible to obtain fallback values when resources are missing. The chain is built by using ResourceBundle’s protected void setParent(ResourceBundle parent) method. getBundle() builds the chain by iterating over the candidate bundle names that can be obtained by successively removing variant, country, and language (each time with the preceding “_”) from the complete resource bundle name of the result resource bundle. NOTE: Candidate bundle names where the final component is an empty string are omitted. With each candidate bundle name, getBundle() tries to instantiate a resource bundle, as just described. If it succeeds, it calls the previously instantiated resource bundle’s setParent() method with the new resource bundle, unless the previously instantiated resource bundle already has a nonnull parent. NOTE: getBundle() caches instantiated resource bundles and may return the same resource bundle instance multiple times. CHAPTER 9: Discovering Additional Utility APIs 403 ResourceBundle declares various methods for accessing a resource bundle’s resources. For example, Object getObject(String key) gets an object for the given key from this resource bundle or one of its parent bundles. getObject() first tries to obtain the object from this resource bundle using the protected abstract handleGetObject() method, which is implemented by concrete subclasses of ResourceBundle (such as PropertyResourceBundle). If handleGetObject() returns null, and if a nonnull parent resource bundle exists, getObject() calls the parent’s getObject() method. If still not successful, it throws MissingResourceException. Two other resource-access methods are String getString(String key) and String[] getStringArray(String key). These convenience methods are wrappers for (String) getObject(key) and (String[]) getObject(key). NOTE: Java version 6 introduced numerous ResourceBundle enhancements. Enhancements range from new clearCache() methods for removing resource bundles from the cache to a new nested Control class whose methods let you take control of the resource bundle search order, and even load resource bundles from another source (such as an XML file). Although Android does not support these enhancements at the time of writing, this may change in a future release of Android. I explore the enhancements to ResourceBundle in Beginning Java SE 6 Platform: From Novice to Professional. Property Resource Bundles A property resource bundle is a resource bundle that is backed by a properties file, a text file (with a .properties extension) that stores textual elements as a series of key=value entries. The key is a nonlocalized identifier that an application uses to obtain the localized value. NOTE: Properties files are accessed via instances of the java.util.Properties class. In Chapter 8, I mentioned that the Preferences API (discussed later in this chapter) has made Properties largely obsolete. Property resource bundles prove that the Properties class is not entirely obsolete. PropertyResourceBundle, a concrete subclass of ResourceBundle, manages property resource bundles. You should rarely (if ever) need to work with this subclass. Instead, for maximum portability, you should only work with ResourceBundle, as Listing 9–10 demonstrates. CHAPTER 9: Discovering Additional Utility APIs 404 Listing 9–10. Accessing a localized elevator entry in game resource bundles import java.util.ResourceBundle; public class PropertyResourceBundleDemo { public static void main(String[] args) { ResourceBundle resources = ResourceBundle.getBundle("game"); System.out.println("elevator = " + resources.getString("elevator")); } } Listing 9–10 refers to ResourceBundle instead of PropertyResourceBundle, which lets you easily migrate to ListResourceBundle as necessary. I use getString() instead of getObject() for convenience; text resources are stored in textual properties files. When you run this application, it generates the following output: Exception in thread "main" java.util.MissingResourceException: Can't find bundle for base name game, locale en_US at java.util.ResourceBundle.throwMissingResourceException(Unknown Source) at java.util.ResourceBundle.getBundleImpl(Unknown Source) at java.util.ResourceBundle.getBundle(Unknown Source) at PropertyResourceBundleDemo.main(PropertyResourceBundleDemo.java:7) This exception is thrown because no property resource bundles exist. You can easily remedy this situation by copying Listing 9–11 into a game.properties file, which is the basis for a property resource bundle. Listing 9–11. A fallback game.properties resource bundle elevator=elevator Assuming that game.properties is located in the same directory as PropertyResourceBundleDemo.class, execute java PropertyResourceBundleDemo and you will see the following output: elevator = elevator Because my locale is en_US, getBundle() first tries to load game_en_US.properties. Because this file does not exist, getBundle() tries to load game_en.properties. Because this file does not exist, getBundle() tries to load game.properties, and succeeds. Copy Listing 9–12 into a game_en_GB.properties file. Listing 9–12. A game resource bundle for the en_GB locale elevator=lift Execute java -Duser.language=en -Duser.country=GB PropertyResourceBundleDemo. This time, you should see the following output: elevator = lift With the locale set to en_GB, getBundle() first tries to load game_en_GB.properties, and succeeds. This book was purchased by harshamv@variable3.comCHAPTER 9: Discovering Additional Utility APIs 405 Comment out elevator = lift by prepending a # character to this line (as in #elevator = lift). Then execute java -Duser.language=en -Duser.country=GB PropertyResourceBundleDemo and you should see the following output: elevator = elevator Although getBundle() loaded game_en_GB.properties, getString() (via getObject()) could not find an elevator entry. As a result, getString()/getObject() searched the parent resource bundle chain, encountering game.properties’ elevator=elevator entry, whose elevator value was subsequently returned. NOTE: A common reason for getString() throwing MissingResourceException in a property resource bundle context is forgetting to append .properties to a properties file’s name. List Resource Bundles A list resource bundle is a resource bundle that is backed by a classfile, which describes a concrete subclass of ListResourceBundle (an abstract subclass of ResourceBundle). As a result, list resource bundles can store binary data (such as images or audio) as well as text. In contrast, property resource bundles can only store text. NOTE: If a property resource bundle and a list resource bundle have the same complete resource bundle name, the list resource bundle takes precedence over the property resource bundle. For example, when getBundle() is confronted with game_en.properties and game_en.class, it loads game_en.class instead of game_en.properties. Listing 9–13 demonstrates a list resource bundle by presenting a flags_en_CA class that extends ListResourceBundle. Listing 9–13. A resource bundle containing a small Canadian flag image and English/French text import java.awt.Toolkit; import java.util.ListResourceBundle; public class flags_en_CA extends ListResourceBundle { private byte image[] = { (byte) 137, (byte) 80, (byte) 78, (byte) 71, (byte) 13, (byte) 10, (byte) 26, (byte) 10, CHAPTER 9: Discovering Additional Utility APIs 406 (byte) 0, (byte) 0, // ... (byte) 0, (byte) 0, (byte) 73, (byte) 69, (byte) 78, (byte) 68, (byte) 174, (byte) 66, (byte) 96, (byte) 130 }; private Object[][] contents = { { "flag", Toolkit.getDefaultToolkit().createImage(image) }, { "msg", "Welcome to Canada! | Bienvenue vers le Canada !" }, { "title", "CANADA | LA CANADA" } }; public Object[][] getContents() { return contents; } } Listing 9–13’s flags_en_CA class describes a list resource bundle whose base name is flags and whose locale designation is en_CA. This class’s image array stores a Portable Network Graphics (PNG)-based sequence of byte integers that describes an image of the Canadian flag, contents stores key/value pairs, and getContents() returns contents. NOTE: For brevity, Listing 9–13 does not present the complete image array with Canadian flag image data. You must obtain flags_en_CA.java from this book’s companion code file (see the book’s introduction for instructions on how to obtain this file) to get the complete listing. The first key/value pair consists of a key named flag (which will be passed to ResourceBundle’s getObject() method) and an instance of the java.awt.Image class. This instance represents the flag image and is obtained with the help of the java.awt.Toolkit class and its createImage() utility method. NOTE: AWT stands for Abstract Window Toolkit, a windowing toolkit that makes it possible to create crude user interfaces consisting of windows, buttons, text fields, and so on. The AWT was released as part of Java version 1.0 in 1995, and continues to be part of Java’s standard class library. The AWT is not supported by Android. Listing 9–14 shows you how to load the default flags_en_CA list resource bundle (or another list resource bundle via command-line arguments) and display its flag and text. CHAPTER 9: Discovering Additional Utility APIs 407 Listing 9–14. Obtaining and displaying a list resource bundle’s flag image and text import java.awt.EventQueue; import java.awt.Image; import java.util.Locale; import java.util.ResourceBundle; import javax.swing.ImageIcon; import javax.swing.JOptionPane; public class ListResourceBundleDemo { public static void main(String[] args) { Locale l = Locale.CANADA; if (args.length == 2) l = new Locale(args[0], args[1]); final ResourceBundle resources = ResourceBundle.getBundle("flags", l); Runnable r = new Runnable() { public void run() { Image image = (Image) resources.getObject("flag"); String msg = resources.getString("msg"); String title = resources.getString("title"); ImageIcon ii = new ImageIcon(image); JOptionPane.showMessageDialog(null, msg, title, JOptionPane.PLAIN_MESSAGE, ii); } }; EventQueue.invokeLater(r); } } Listing 9–14’s main() method begins by selecting CANADA as its default Locale. If it detects that two arguments were passed on the command line, main() assumes that the first argument is the language code and the second argument is the country code, and creates a new Locale object based on these arguments as its default locale. main() next attempts to load a list resource bundle by passing the flags base name and the previously chosen Locale object to ResourceBundle’s getBundle() method. Assuming that MissingResourceException is not thrown, main() creates a runnable task on which to load resources from the list resource bundle and display them graphically. main() relies on a windowing toolkit known as Swing to present a simple user interface that displays the flag and text. Because Swing is single threaded, where everything runs on a special thread known as the event-dispatching thread, it is important that all Swing- based operations occur on this thread. EventQueue.invokeLater() makes this happen. CHAPTER 9: Discovering Additional Utility APIs 408 NOTE: Swing is built on top of the AWT and provides many sophisticated features. This windowing toolkit was officially released as part of Java version 1.2, and continues to be part of Java’s standard class library. Swing is not supported by Android. If you would like to learn more about Swing, I recommend the definitive Java Swing, Second Edition, by Marc Loy, Robert Eckstein, David Wood, James Elliott, and Brian Cole (O’Reilly Media, 2002; ISBN: 0596004087). Shortly after EventQueue.invokeLater() is executed on the main thread, the event- dispatching thread starts running and executes the runnable task. This task first obtains the Image object from the list resource bundle by passing flag to getObject() and casting this method’s return value to Image. The task then obtains the msg and title strings by passing these keys to getString(), and converts the Image object to a javax.swing.ImageIcon instance. This instance is required by the subsequent JOptionPane.showMessageDialog() method call, which presents a simple message-oriented dialog box. NOTE: JOptionPane is a Swing component that makes it easy to display a standard dialog box that prompts the user to enter a value or informs the user of something important. Now that you understand how ListResourceBundleDemo works, obtain the complete flags_en_CA.java source file, compile this source code (javac flags_en_CA.java) and Listing 9–14 (javac ListResourceBundleDemo.java), and execute the application (java ListResourceBundleDemo). Figure 9–1 shows the resulting user interface on Windows XP. Figure 9–1. An almost completely localized dialog box (OK is not localized) displaying Canada-specific resources on Windows XP NOTE: I obtained the language translations for this section’s examples from Yahoo! Babel Fish (http://babelfish.yahoo.com/), an online text translation service. This book’s accompanying code file also contains flags_fr_FR.java, which presents resources localized for the France locale. After compiling this source file, execute java ListResourceBundleDemo fr FR and you will see Figure 9–2. CHAPTER 9: Discovering Additional Utility APIs 409 Figure 9–2. An almost completely localized dialog box displaying France-specific resources on Windows XP Finally, this book’s accompanying code file also contains flags_ru_RU.java, which presents resources localized for the Russia locale. After compiling this source file, execute java ListResourceBundleDemo ru RU and you will see Figure 9–3. Figure 9–3. An almost completely localized dialog box displaying Russia-specific resources on Windows XP NOTE: You will need to ensure that appropriate Cyrillic fonts are installed to view the Russian text. Also, because I stored Russian characters verbatim (and not as Unicode escape sequences, such as '\u0041'), flags_ru_RU.java is a Unicode file and must be compiled via javac – encoding Unicode flags_ru_RU.java. Break Iterators Internationalized text-processing applications (such as word processors) need to detect logical boundaries within the text they are manipulating. For example, a word processor needs to detect these boundaries when highlighting a character, selecting a word to cut to the clipboard, moving the caret (text insertion point indicator) to the start of the next sentence, and wrapping a word at the end of a line. Java provides the Break Iterator API with its abstract java.text.BreakIterator entry- point class to detect text boundaries. BreakIterator declares the following factory methods for obtaining break iterators that detect character, word, sentence, and line boundaries:  static BreakIterator getCharacterInstance()  static BreakIterator getWordInstance()  static BreakIterator getSentenceInstance()  static BreakIterator getLineInstance() CHAPTER 9: Discovering Additional Utility APIs 410 Each of these factory methods returns a break iterator for the default locale. If you need a break iterator for a specific locale, you can call the following factory methods:  static BreakIterator getCharacterInstance(Locale locale)  static BreakIterator getWordInstance(Locale locale)  static BreakIterator getSentenceInstance(Locale locale)  static BreakIterator getLineInstance(Locale locale) Each of these factory methods throws NullPointerException when its locale argument is null. BreakIterator’s locale-sensitive factory methods might not support every locale. For this reason, you should only pass Locale objects that are also stored in the array returned from this class’s static Locale[] getAvailableLocales() method (which is also declared in other entry-point classes) to the aforementioned factory methods—this array contains at least Locale.US. Check out Listing 9–15. Listing 9–15. Obtaining BreakIterator’s supported locales and passing the first locale (possibly Locale.US) to getCharacterInstance(Locale) Locale[] supportedLocales = BreakIterator.getAvailableLocales(); BreakIterator bi = BreakIterator.getCharacterInstance(supportedLocales[0]); A BreakIterator instance has an imaginary cursor that points to the current boundary within a text string. This cursor position can be interrogated and the cursor moved from boundary to boundary with the help of the following BreakIterator methods:  abstract int current() returns the text boundary that was most recently returned by next(), next(int), previous(), first(), last(), following(int), or preceding(int). If any of these methods returns BreakIterator.DONE because either the first or the last text boundary has been reached, current() returns the first or last text boundary depending on which one was reached.  abstract int first() returns the first text boundary. The iterator’s current position is set to this boundary.  abstract int following(int offset) returns the first text boundary following the specified character offset. If offset equals the last text boundary, following(int) returns BreakIterator.DONE and the iterator’s current position is unchanged. Otherwise, the iterator’s current position is set to the returned text boundary. The value returned is always greater than offset or BreakIterator.DONE.  abstract int last() returns the last text boundary. The iterator’s current position is set to this boundary.  abstract int next() returns the text boundary following the current boundary. If the current boundary is the last text boundary, next() returns BreakIterator.DONE and the iterator’s current position is CHAPTER 9: Discovering Additional Utility APIs 411 unchanged. Otherwise, the iterator’s current position is set to the boundary following the current boundary.  abstract int next(int n) returns the nth text boundary from the current boundary. If either the first or the last text boundary has been reached, next(int) returns BreakIterator.DONE and the current position is set to either the first or last text boundary depending on which one is reached. Otherwise, the iterator’s current position is set to the new text boundary.  int preceding(int offset) returns the last text boundary preceding the specified character offset. If offset equals the first text boundary, preceding(int) returns BreakIterator.DONE and the iterator’s current position is unchanged. Otherwise, the iterator’s current position is set to the returned text boundary. The returned value is always less than offset or equals BreakIterator.DONE. (This method was added to BreakIterator in Java version 1.2. It could not be declared abstract because abstract methods cannot be added to existing classes; such methods would also have to be implemented in subclasses that might be inaccessible.)  abstract int previous() returns the text boundary preceding the current boundary. If the current boundary is the first text boundary, previous() returns BreakIterator.DONE and the iterator’s current position is unchanged. Otherwise, the iterator’s current position is set to the boundary preceding the current boundary. Figure 9–4 reveals that characters are located between boundaries, boundaries are zero-based, and the last boundary is the length of the string. Figure 9–4. JAVA’s character boundaries as reported by the next() and previous() methods BreakIterator also declares a void setText(String newText) method that identifies newText as the text to be iterated over. This method resets the cursor position to the beginning of this string. Listing 9–16 shows you how to use a character-based break iterator to iterate over a string’s characters in a locale-independent manner. Listing 9–16. Iterating over English/US and Arabic/Saudi Arabia strings import java.text.BreakIterator; import java.util.Locale; public class BreakIteratorDemo yCHAPTER 9: Discovering Additional Utility APIs 412 { public static void main(String[] args) { BreakIterator bi = BreakIterator.getCharacterInstance(Locale.US); bi.setText("JAVA"); dumpPositions(bi); bi = BreakIterator.getCharacterInstance(new Locale("ar", "SA")); bi.setText("\u0631\u0641\u0651"); dumpPositions(bi); } static void dumpPositions(BreakIterator bi) { int boundary = bi.first(); while (boundary != BreakIterator.DONE) { System.out.print(boundary + " "); boundary = bi.next(); } System.out.println(); } } The main() method first obtains a character-based break iterator for the United States locale. main() then calls the iterator’s setText() method to specify JAVA as the text to be iterated over. Iteration occurs in the dumpPositions() method. After calling first() to obtain the first boundary, this method uses a while loop to output the boundary and move to the next boundary (via next()) while the current boundary does not equal BreakIterator.DONE. Because character iteration is straightforward for English words, main() next obtains a character-based break iterator for the Saudi Arabia locale, and uses this iterator to iterate over the characters in Figure 9–5’s Arabic version of “shelf” (as in shelf of books). Figure 9–5. The letters and diacritic making up the Arabic equivalent of “shelf” are written from right to left. In Arabic, the word “shelf” consists of letters resh and pe, and diacritic shadda. A diacritic is an ancillary glyph, or mark on paper or other writing medium, added to a letter, or basic glyph. Shadda, which is shaped like a small written Latin w, indicates gemination (consonant doubling or extra length), which is phonemic (the smallest identifiable discrete unit of sound employed to form meaningful contrasts between CHAPTER 9: Discovering Additional Utility APIs 413 utterances) in Arabic. Shadda is written above the consonant that is to be doubled, which happens to be pe in this example. When you run this application, it generates the following output: 0 1 2 3 4 0 1 3 The first output line reveals Figure 9–4’s character boundaries for the word JAVA. The second output line (0 comes before resh, 1 comes before pe) implies that you cannot move an Arabic word processor’s caret on the screen once for every Unicode character. Instead, it is moved once for every user character, a logical character that can be composed of multiple Unicode characters, such as pe (\u0641) and shadda (\u0651). NOTE: For examples of break iterators that iterate over words, sentences, and lines, check out the “Detecting Text Boundaries” section (http://download.oracle.com/docs/cd/E17409_01/javase/tutorial/i18n/text/ boundaryintro.html) in The Java Tutorials’ Internationalization trail. Collators Applications perform string comparisons while sorting text. When an application targets English-oriented users, String’s compareTo() method is probably sufficient for comparing strings. However, this method’s binary comparison of each string’s Unicode characters is not reliable for languages where the relative order of their characters does not correspond to the Unicode values of these characters. French is one example. Java provides the Collator API with its abstract java.text.Collator entry-point class for making reliable comparisons. Collator declares the following factory methods for obtaining collators:  static Collator getInstance()  static Collator getInstance(Locale locale) The first factory method obtains a collator for the default locale; the second factory method throws NullPointerException when its locale argument is null. As with BreakIterator, you should only pass Locale objects that are also stored in the array returned from Collator’s static Locale[] getAvailableLocales() method to the second factory method. Listing 9–17 shows you how to use a collator to perform comparisons so that French words that differ only in terms of accented characters are sorted into the correct order. Listing 9–17. Using a collator to correctly order French words in the France locale import java.text.Collator; import java.util.Arrays; import java.util.Locale; CHAPTER 9: Discovering Additional Utility APIs 414 public class CollatorDemo { public static void main(String[] args) { Collator en_USCollator = Collator.getInstance(Locale.US); Collator fr_FRCollator = Collator.getInstance(Locale.FRANCE); String[] words = { "côte", "coté", "côté", "cote" }; Arrays.sort(words, en_USCollator); for (String word: words) System.out.println(word); System.out.println(); Arrays.sort(words, fr_FRCollator); for (String word: words) System.out.println(word); } } In Listing 9–17, each of the four words being sorted has a different meaning. For example, côte means coast and côté means side. When you run this application, it generates the following output (I am showing this output as it appears in the Windows XP Notepad editor): cote coté côte côté cote côte coté côté The first four output lines show the order in which these words are sorted according to the en_US locale. This ordering is not correct because it does not account for accents. In contrast, the final four output lines show the correct order when the words are sorted according to the fr_FR locale. Words are compared as if none of the characters contain accents, and then equal words are compared from right to left for accents. NOTE: Learn about Collator’s java.text.RuleBasedCollator subclass for creating custom collators when predefined collation rules do not meet your needs, and about improving collation performance via java.text.CollationKey and Collator’s CollationKey getCollationKey(String source) method by reading the “Comparing Strings” section (http://download.oracle.com/docs/cd/E17409_01/javase/tutorial/i18n/text/ collationintro.html) in The Java Tutorials’ Internationalization trail. This book was purchased by harshamv@variable3.comCHAPTER 9: Discovering Additional Utility APIs 415 Dates, Time Zones, and Calendars Internationalized applications must properly handle dates, time zones, and calendars. A date is a recorded temporal moment, a time zone is a set of geographical regions that share a common number of hours relative to Greenwich Mean Time (GMT), and a calendar is a system of organizing the passage of time. NOTE: GMT identifies the standard geographical location from where all time is measured. UTC, which stands for Coordinated Universal Time, is often specified in place of GMT. Java version 1.0 introduced the java.util.Date class as its first attempt to describe calendars. However, Date was not amenable to internationalization because of its English-oriented nature, and because of its inability to represent dates prior to midnight January 1, 1970 GMT, which is known as the Unix epoch. Date was eventually refactored to make it more useful by allowing Date instances to represent dates prior to the epoch as well as after the epoch, and by deprecating most of this class’s constructors and methods—deprecated methods have been replaced by more appropriate API classes. Table 9–3 describes the more useful Date class. Table 9–3. Date Constructors and Methods Method Description Date() Allocate a Date object and initialize it to the current time by calling System.currentTimeMillis(). Date(long date) Allocate a Date object and initialize it to the time represented by date milliseconds. A negative value indicates a time before the epoch, 0 indicates the epoch, and a positive value indicates a time after the epoch. boolean after (Date date) Return true when this date occurs after date. This method throws NullPointerException when date is null. boolean before (Date date) Return true when this date occurs before date. This method throws NullPointerException when date is null. Object clone() Return a copy of this object. int compareTo (Date date) Compare this date with date. Return 0 when this date equals date, a negative value when this date comes before date, and a positive value when this date comes after date. This method throws NullPointerException when date is null. boolean equals (Object obj) Compare this date with the Date object represented by obj. Return true if and only if obj is not null and is a Date object that represents the same point in time (to the millisecond) as this date. CHAPTER 9: Discovering Additional Utility APIs 416 Method Description long getTime() Return the number of milliseconds that must elapse before the epoch (a negative value) or have elapsed since the epoch (a positive value). int hashCode() Return this date’s hash code. The result is the exclusive OR of the two halves of the long integer value returned by getTime(). That is, the hash code is the value of expression (int) (this.getTime()^(this.getTime()>>>32)). void setTime (long time) Set this date to represent the point in time specified by time milliseconds (a negative value refers to before the epoch; a positive value refers to after the epoch). String toString() Return a String object containing this date’s representation as dow mon dd hh:mm:ss zzz yyyy, where dow is the day of the week (Sun, Mon, Tue, Wed, Thu, Fri, Sat), mon is the month (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec), dd is the two decimal digit day of the month (01 through 31), hh is the two decimal digit hour of the day (00 through 23), mm is the two decimal digit minute within the hour (00 through 59), ss is the two decimal digit second within the minute (00 through 61, where 60 and 61 account for leap seconds), zzz is the (possibly empty) time zone (and may reflect daylight saving time), and yyyy is the four decimal digit year. Listing 9–18 provides a small demonstration of the Date class. Listing 9–18. Exploring the Date class import java.util.Date; class DateDemo { public static void main(String[] args) { Date now = new Date(); System.out.println(now); Date later = new Date(now.getTime()+86400); System.out.println(later); System.out.println(now.after(later)); System.out.println(now.before(later)); } } Listing 9–18’s main() method creates a pair of Date objects (now and later) and outputs their dates, formatted according to Date’s implicitly called toString() method. main() then demonstrates after() and before(), proving that now comes before later, which is one second in the future. When you run this application, it generates output similar to the following: Sun Jul 25 18:36:20 CDT 2010 Sun Jul 25 18:37:47 CDT 2010 false true CHAPTER 9: Discovering Additional Utility APIs 417 The toString() method’s description and Listing 9–18’s output reveal that a time zone is part of a date. Java provides the abstract java.util.TimeZone entry-point class for obtaining TimeZone instances. This class declares a pair of factory methods for obtaining TimeZone instances:  static TimeZone getDefault()  static TimeZone getTimeZone(String ID) The latter method returns a TimeZone instance for the time zone whose String identifier (such as "CST") is passed to ID. NOTE: Some time zones take into account daylight saving time, the practice of temporarily advancing clocks so that afternoons have more daylight and mornings have less; for example, Central Daylight Time (CDT). Check out Wikipedia’s “Daylight saving time” entry (http://en.wikipedia.org/wiki/Daylight_saving_time) to learn more about daylight saving time. If you need to introduce a new time zone or modify an existing time zone, perhaps to deal with changes to a time zone’s daylight saving time policy, you can work directly with TimeZone’s java.util.SimpleTimeZone concrete subclass. SimpleTimeZone describes a raw offset from GMT and provides rules for specifying the start and end of daylight saving time. Java version 1.1 introduced the Calendar API with its abstract java.util.Calendar entry-point class as a replacement for Date. Calendar is intended to represent any kind of calendar. However, time constraints meant that only the Gregorian calendar could be implemented (via the concrete java.util.GregorianCalendar subclass) for version 1.1. NOTE: Java version 1.4 introduced support for the Thai Buddhist calendar via an internal class that subclasses Calendar. Java version 6 introduced support for the Japanese Imperial Era calendar via the package-private java.util.JapaneseImperialCalendar class, which also subclasses Calendar. For an in-depth look at the Japanese Imperial Era calendar, and to explore an example that presents this calendar graphically, check out this topic in Beginning Java SE 6 Platform: From Novice to Professional. Calendar declares the following factory methods for obtaining calendars:  static Calendar getInstance()  static Calendar getInstance(Locale locale)  static Calendar getInstance(TimeZone zone)  static Calendar getInstance(TimeZone zone, Locale locale) The first and third methods return calendars for the default locale; the second and fourth methods take the specified locale into account. Also, calendars returned by the first two CHAPTER 9: Discovering Additional Utility APIs 418 methods are based on the current time in the default time zone; calendars returned by the last two methods are based on the current time in the specified time zone. Calendar declares various constants, including YEAR, MONTH, DAY_OF_MONTH, DAY_OF_WEEK, LONG, and SHORT. These constants identify the year (four digits), month (0 represents January), current month day (1 through the month’s last day), and current weekday (1 represents Sunday) calendar fields, and display styles (such as January versus Jan). The first four constants are used with Calendar’s various set() methods to set calendar fields to specific values (set the year field to 2010, for example). They are also used with Calendar’s int get(int field) method to return field values, along with other field- oriented methods, such as void clear(int field) (unset a field). The latter two constants are used with Calendar’s String getDisplayName(int field, int style, Locale locale) and Map getDisplayNames(int field, int style, Locale locale) methods, which return short (Jan, for example) or long (January, for example) localized String representations of various field values. Listing 9–19 shows you how to use various Calendar constants and methods to output calendar pages according to the en_US and fr_FR locales. Listing 9–19. Outputting calendar pages import java.util.Calendar; import java.util.Iterator; import java.util.Locale; import java.util.Map; import java.util.Set; public class CalendarDemo { public static void main(String[] args) { if (args.length < 2) { System.err.println("usage: java CalendarDemo yyyy mm [f|F]"); return; } try { int year = Integer.parseInt(args[0]); int month = Integer.parseInt(args[1]); Locale locale = Locale.US; if (args.length == 3 && args[2].equalsIgnoreCase("f")) locale = Locale.FRANCE; showPage(year, month, locale); } catch (NumberFormatException nfe) { System.err.print(nfe); } } static void showPage(int year, int month, Locale locale) { if (month < 1 || month > 12) throw new IllegalArgumentException("month [" + month + "] out of " + CHAPTER 9: Discovering Additional Utility APIs 419 "range [1, 12]"); Calendar cal = Calendar.getInstance(locale); cal.set(Calendar.YEAR, year); cal.set(Calendar.MONTH, --month); cal.set(Calendar.DAY_OF_MONTH, 1); displayMonthAndYear(cal, locale); displayWeekdayNames(cal, locale); int daysInMonth = cal.getActualMaximum(Calendar.DAY_OF_MONTH); int firstRowGap = cal.get(Calendar.DAY_OF_WEEK)-1; // 0 = Sunday for (int i = 0; i < firstRowGap; i++) System.out.print(" "); for (int i = 1; i <= daysInMonth; i++) { if (i < 10) System.out.print(' '); System.out.print(i); if ((firstRowGap+i)%7 == 0) System.out.println(); else System.out.print(' '); } System.out.println(); } static void displayMonthAndYear(Calendar cal, Locale locale) { System.out.println(cal.getDisplayName(Calendar.MONTH, Calendar.LONG, locale) + " " + cal.get(Calendar.YEAR)); } static void displayWeekdayNames(Calendar cal, Locale locale) { Map weekdayNamesMap; weekdayNamesMap = cal.getDisplayNames(Calendar.DAY_OF_WEEK, Calendar.SHORT, locale); String[] names = new String[weekdayNamesMap.size()]; int[] indexes = new int[weekdayNamesMap.size()]; Set> weekdayNamesEntries; weekdayNamesEntries = weekdayNamesMap.entrySet(); Iterator> iter; iter = weekdayNamesEntries.iterator(); while (iter.hasNext()) { Map.Entry entry = iter.next(); names[entry.getValue()-1] = entry.getKey(); indexes[entry.getValue()-1] = entry.getValue(); } for (int i = 0; i < names.length; i++) for (int j = i; j < names.length; j++) if (indexes[j] == i+1) { System.out.print(names[j].substring(0, 2) + " "); continue; } System.out.println(); } } CHAPTER 9: Discovering Additional Utility APIs 420 Listing 9–19 is pretty straightforward, with the exception of displayWeekdayNames(). This method calls Calendar’s getDisplayNames() method to return a map of localized weekday names. Instead of returning a map where the keys are Integers and the values are localized Strings, this map’s keys are the localized Strings. This would be fine if the keys were ordered (as in Sunday first and Saturday last, or lundi first and dimanche last). However, they are not ordered. To output these names in order, it is necessary to obtain a set of map entries, iterate over these entries and populate parallel arrays, and then iterate over these arrays to output the weekday names. NOTE: A French calendar begins the week on lundi (Monday) and ends it on dimanche (Sunday). However, Calendar does not take this ordering into account. Specify java CalendarDemo 2010 7 and you will see the following calendar page for the en_US locale: July 2010 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 If you would like to see this page in the fr_FR locale, specify java CalendarDemo 2010 7 f or java CalendarDemo 2010 7 F: juillet 2010 di lu ma me je ve sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 NOTE: Calendar declares a Date getTime() method that returns a calendar’s time representation as a Date instance. Calendar also declares a void setTime(Date date) method that sets a calendar’s time representation to the specified date. Joda Time (http://joda-time.sourceforge.net/), the Java date and time API, offers a high-quality replacement for Java’s date and time classes. You might prefer to use these classes as an alternative. CHAPTER 9: Discovering Additional Utility APIs 421 Formatters Internationalized applications do not present unformatted numbers (including currencies and percentages), dates, and messages to the user. These items must be formatted according to the user’s locale so they appear meaningful to the user. To help with formatting, Java provides the abstract java.text.Format class and various subclasses. NOTE: Format defines the programming interface for formatting locale-sensitive objects into Strings via its format() methods, and for parsing Strings back into objects via its parseObject() methods. For brevity, I ignore the parseObject() methods. Number Formatters The abstract java.text.NumberFormat entry-point class (a Format subclass) declares the following factory methods to return formatters that format numbers as currencies, integers, numbers with decimal points, and percentages (and also to parse such values):  static NumberFormat getCurrencyInstance()  static NumberFormat getCurrencyInstance(Locale locale)  static NumberFormat getIntegerInstance()  static NumberFormat getIntegerInstance(Locale locale)  static NumberFormat getInstance()  static NumberFormat getInstance(Locale locale)  static NumberFormat getNumberInstance()  static NumberFormat getNumberInstance(Locale locale)  static NumberFormat getPercentInstance()  static NumberFormat getPercentInstance(Locale locale) The getInstance() and getInstance(Locale) factory methods are equivalent to getNumberInstance() and getNumberInstance(Locale). They are present as a shorthand convenience to the longer-named getNumberInstance() methods. Listing 9–20 shows you how to obtain and use number formatters to format numbers as currencies, integers, numbers with decimals points, and percentages for various locales. Listing 9–20. Formatting numbers as currencies, integers, numbers with decimal points, and percentages import java.text.NumberFormat; import java.util.Locale; public class NumberFormatDemo { CHAPTER 9: Discovering Additional Utility APIs 422 public static void main(String[] args) { System.out.println("Unformatted: " + 9875432.25); formatCurrencies(Locale.US, 98765432.25); formatCurrencies(Locale.FRANCE, 98765432.25); formatCurrencies(Locale.GERMANY, 98765432.25); System.out.println(); System.out.println("Unformatted: " + 123456789.0); formatIntegers(Locale.US, 123456789.0); formatIntegers(Locale.FRANCE, 123456789.0); formatIntegers(Locale.GERMANY, 123456789.0); System.out.println(); System.out.println("Unformatted: " + 6751.326); formatNumbers(Locale.US, 6751.326); formatNumbers(Locale.FRANCE, 6751.326); formatNumbers(Locale.GERMANY, 6751.326); System.out.println(); System.out.println("Unformatted: " + 0.85); formatPercentages(Locale.US, 0.85); formatPercentages(Locale.FRANCE, 0.85); formatPercentages(Locale.GERMANY, 0.85); } static void formatCurrencies(Locale locale, double amount) { NumberFormat nf = NumberFormat.getCurrencyInstance(locale); System.out.println(locale + ": " + nf.format(amount)); } static void formatIntegers(Locale locale, double amount) { NumberFormat nf = NumberFormat.getIntegerInstance(locale); System.out.println(locale + " : " + nf.format(amount)); } static void formatNumbers(Locale locale, double amount) { NumberFormat nf = NumberFormat.getNumberInstance(locale); System.out.println(locale + ": " + nf.format(amount)); } static void formatPercentages(Locale locale, double amount) { NumberFormat nf = NumberFormat.getPercentInstance(locale); System.out.println(locale + ": " + nf.format(amount)); } } Listing 9–20 uses a double instead of a BigDecimal object to represent 9875432.25 as a currency, for simplicity and because this value can be represented exactly as a double. When you run this application, it generates output that is shown with the Windows XP Notepad editor window (so that the Euro currency symbol can be seen) in Figure 9–6. CHAPTER 9: Discovering Additional Utility APIs 423 Figure 9–6. Unformatted and formatted numeric output for the US, France, and Germany locales NumberFormat declares void setMaximumFractionDigits(int newValue), void setMaximumIntegerDigits(int newValue), void setMinimumFractionDigits(int newValue), and void setMinimumIntegerDigits(int newValue) methods to limit the number of digits that are allowed in a formatted number’s integer or fraction. These methods are helpful for aligning numbers, as Listing 9–21 demonstrates. Listing 9–21. Aligning numbers NumberFormat nf = NumberFormat.getInstance(); System.out.println(nf.format(123.4567)); // Output: 123.457 nf.setMaximumIntegerDigits(10); nf.setMinimumIntegerDigits(6); nf.setMaximumFractionDigits(2); nf.setMinimumFractionDigits(2); System.out.println(nf.format(123.4567)); // Output: 000,123.46 System.out.println(nf.format(80978.3)); // Output : 080,978.30 This code fragment specifies that a number’s integer portion cannot exceed ten digits, but must have a minimum of six digits. Leading zeros are output to meet the minimum. The code fragment reveals that the fraction is rounded. A concrete subclass of NumberFormat might enforce an upper limit on the value passed to setMaximumFractionDigits(int), setMaximumIntegerDigits(int), setMinimumFractionDigits(int), or setMinimumIntegerDigits(int). Call getMaximumFractionDigits(), getMaximumIntegerDigits(), getMinimumFactionDigits(), or getMinimumIntegerDigits() to find out if the value you specified has been accepted. CHAPTER 9: Discovering Additional Utility APIs 424 NOTE: If you ever need to create customized number formatters, you will find yourself working with NumberFormat’s java.text.DecimalFormat subclass and this subclass’s java.text.DecimalFormatSymbols companion class. The “Customizing Formats” section (http://download.oracle.com/docs/cd/E17409_01/javase/tutorial/i18n/forma t/decimalFormat.html) in The Java Tutorials’ Internationalization trail introduces you to these classes. Date Formatters The abstract java.text.DateFormat entry-point class (a Format subclass) provides access to formatters that format Date instances as dates or time values (and also to parse such values). This class declares the following factory methods:  static DateFormat getDateInstance()  static DateFormat getDateInstance(int style)  static DateFormat getDateInstance(int style, Locale locale)  static DateFormat getDateTimeInstance()  static DateFormat getDateTimeInstance(int dateStyle, int timeStyle)  static DateFormat getDateTimeInstance(int dateStyle, int timeStyle, Locale locale)  static DateFormat getInstance()  static DateFormat getTimeInstance()  static DateFormat getTimeInstance(int style)  static DateFormat getTimeInstance(int style, Locale locale) The getDateInstance() factory methods’ formatters generate only date information, the getTimeInstance() factory methods’ formatters generate only time information, and the getDateTimeInstance() factory methods’ formatters generate date and time information. The dateStyle and timeStyle fields determine how that information will be presented according to the following DateFormat constants:  SHORT is completely numeric, such as 12.13.52 or 3:30pm  MEDIUM is longer, such as Jan 12, 1952  LONG is longer, such as January 12, 1952 or 3:30:32pm  FULL is pretty completely specified, such as Tuesday, April 12, 1952 AD or 3:30:42pm PST. Listing 9–22 shows you how to format a Date instance that represents the Unix epoch according to the local time zone and the UTC time zone. This book was purchased by harshamv@variable3.comCHAPTER 9: Discovering Additional Utility APIs 425 Listing 9–22. Formatting the Unix epoch import java.text.DateFormat; import java.util.Date; import java.util.Locale; import java.util.TimeZone; public class DateFormatDemo { public static void main(String[] args) { Date d = new Date(0); // Unix epoch System.out.println(d); DateFormat df = DateFormat.getDateTimeInstance(DateFormat.LONG, DateFormat.LONG, Locale.US); System.out.println("Default format: " + df.format(d)); df.setTimeZone(TimeZone.getTimeZone("UTC")); System.out.println("Taking UTC into account: " + df.format(d)); } } When you run this application, it generates the following output for the CST time zone: Wed Dec 31 18:00:00 CST 1969 Default format: December 31, 1969 6:00:00 PM CST Taking UTC into account: January 1, 1970 12:00:00 AM UTC The Unix epoch, which is represented by passing 0 to the Date(long) constructor, is defined as January 1, 1970 00:00:00 UTC, but the first output line does not indicate this fact. Instead, it shows the epoch in my CST time zone, which is six hours away from GMT/UTC. To show the epoch correctly, I need to obtain the UTC time zone, which I accomplish by passing "UTC" to TimeZone’s getTimeZone(String) factory method, and install this time zone instance into the date formatter with the help of DateFormat’s void setTimeZone(TimeZone zone) method. NOTE: If you ever need to create customized date formatters, you will find yourself working with DateFormat’s java.text.SimpleDateFormat subclass and this subclass’s java.text.DateFormatSymbols companion class. The “Customizing Formats” section (http://download.oracle.com/docs/cd/E17409_01/javase/tutorial/i18n/forma t/decimalFormat.html) and the “Changing Date Format Symbols” section (http://download.oracle.com/docs/cd/E17409_01/javase/tutorial/i18n/forma t/dateFormatSymbols.html) in The Java Tutorials’ Internationalization trail introduce you to these classes. CHAPTER 9: Discovering Additional Utility APIs 426 Message Formatters Applications often display simple and/or compound status and error messages to the user. A simple message consists of static (unchanging) text, whereas a compound message consists of static text and variable (changeable) data. For example, consider the following compound messages, where the underlined text identifies variable data: 10,536 visitors have visited your website since June 16, 2010. Warning: 25 files have been modified in a suspicious manner. Account balance is $10,567.00! For a simple message, you obtain its text from a resource bundle and then display this text to the user. For a compound message, you obtain a pattern (template) for the message from a property resource bundle, pass this pattern along with the variable data to a message formatter to create a simple message, and display this message’s text. A message formatter is an instance of the concrete java.util.MessageFormat class (a Format subclass). (Unlike other APIs, the Message Format API does not have an abstract entry-point class with factory methods for obtaining instances of subclasses.) This class declares the following constructors:  MessageFormat(String pattern) initializes a MessageFormat instance to the specified pattern and the default locale. This constructor throws IllegalArgumentException when pattern is invalid.  MessageFormat(String pattern, Locale locale) initializes a MessageFormat instance to the specified pattern and locale. This constructor throws IllegalArgumentException when pattern is invalid. A pattern consists of static text and placeholders for variable data. Each placeholder is a brace-delimited sequence of a zero-based integer identifier, an optional format type, and an optional format style. Examples include {0} (insert text between braces), {1, date} (insert a date in default style), and {2, number, currency} (insert a currency). For example, the previous set of compound messages can be converted into Listing 9– 23’s patterns for the en_US locale. Listing 9–23. Patterns in an example_en_US.properties file p1 = {0, number, integer} visitors have visited your website since {1, date, long}. p2 = Warning: {0, number, integer} files have been modified in a suspicious manner. p3 = Account balance is {0, number, currency}! The same placeholders can be used in equivalent compound messages localized to another locale, such as Listing 9–24’s fr_FR locale. Listing 9–24. Patterns in an example_fr_FR.properties file p1 = {0, number, integer} visiteurs ont visité votre site Web depuis le {1, date, long}. p2 = Avertissement : {0, number, integer} dossiers ont été modifiés d'une façon soupçonneuse. p3 = L''équilibre de compte est {0, number, currency} ! CHAPTER 9: Discovering Additional Utility APIs 427 NOTE: An apostrophe (also known as a single quote) in a pattern starts a quoted string, in which, for example, '{0, number, currency} is treated as a literal string and is not interpreted as a placeholder by the formatter. To ensure that this placeholder is not treated as a literal string in Listing 9–24, L'équilibre’s single quote must be doubled, which is why L''équilibre appears. After instantiating a MessageFormat instance, where the pattern is obtained from a resource bundle, you typically create an array of Objects and call Message’s inherited String format(Object obj) method (from Format) with this array—passing an array of Objects to a method whose parameter type is Object works because arrays are Objects. When format() is called, it scans the pattern, replacing each placeholder with the corresponding entry in the Objects array. For example, if format() finds a placeholder with integer identifier 0, it causes the zeroth entry in the Objects array to be formatted and then the formatted results to be output. TIP: You might find MessageFormat’s static String format(String pattern, Object... arguments) method convenient for one-time formatting operations. This method is equivalent to executing new MessageFormat(pattern).format(arguments, new StringBuffer(), null).toString() on the default locale. Listing 9–25 demonstrates message formatting in the context of Listings 9–23’s and 9– 24’s properties files and their localized patterns. Listing 9–25. Formatting and outputting compound messages according to the en_US and fr_FR locales import java.text.MessageFormat; import java.util.Calendar; import java.util.Locale; import java.util.ResourceBundle; public class MessageFormatDemo { public static void main(String[] args) { dumpMessages(Locale.US); System.out.println(); dumpMessages(Locale.FRANCE); } static void dumpMessages(Locale locale) { ResourceBundle rb = ResourceBundle.getBundle("example", locale); MessageFormat mf = new MessageFormat(rb.getString("p1"), locale); Calendar cal = Calendar.getInstance(locale); cal.set(Calendar.YEAR, 2010); cal.set(Calendar.MONTH, Calendar.JUNE); cal.set(Calendar.DAY_OF_MONTH, 16); Object[] args = new Object[] { 10536, cal.getTime() }; CHAPTER 9: Discovering Additional Utility APIs 428 System.out.println(mf.format(args)); mf.applyPattern(rb.getString("p2")); args = new Object[] { 25 }; System.out.println(mf.format(args)); mf.applyPattern(rb.getString("p3")); args = new Object[] { 10567.0 }; System.out.println(mf.format(args)); } } Listing 9–25 takes advantage of MessageFormat’s void applyPattern(String pattern) method to override a previous pattern with a new pattern. When you run this application, it generates output that is shown with the Windows XP Notepad editor window (so that the Euro currency symbol can be seen) in Figure 9–7. Figure 9–7. Compound messages formatted for the en_US and fr_FR locales NOTE: Some compound messages contain singular and plural words. For example, Logging 1 message to x.log and Logging 2 messages to x.log reveal singular and plural messages. Although you could specify pattern Logging {0} message(s) to {1}, it is not grammatically correct to state Logging 2 message(s) to x.log. The solution to this problem is to use the concrete java.text.ChoiceFormat class, a subclass of NumberFormat and a partner of MessageFormat, so that you can output Logging 1 message to x.log or Logging 2 messages to x.log depending on the numeric value passed to {0}. To learn how to use ChoiceFormat, check out the “Handling Plurals” section (http://download.oracle.com/docs/cd/E17409_01/javase/tutorial/i18n/forma t/choiceFormat.html) in The Java Tutorials’ Internationalization trail. Preferences API Significant applications have preferences, which are configuration items. Examples include the location and size of the application’s main window, and the locations and names of files that the application most recently accessed. Preferences are persisted to a file, to a database, or to some other storage mechanism so that they will be available to the application the next time it runs. CHAPTER 9: Discovering Additional Utility APIs 429 The simplest approach to persisting preferences is to use the Properties API, which consists of the Properties class. This class persists preferences as a series of key=value entries to text-based properties files. Although properties files are ideal for simple applications with few preferences, they have proven to be problematic with larger applications:  Properties files tend to grow in size and the probability of name collisions among the various keys increases. This problem could be eliminated if properties files stored preferences in a hierarchy, but they are nonhierarchical in nature.  As an application grows in size and complexity, it tends to acquire numerous properties files with each part of the application associated with its own properties file. The names and locations of these properties files must be hard-coded in the application’s source code. Additionally, someone could directly modify these text-based properties files (perhaps inserting gibberish), causing the application that depends upon the modified properties file to crash unless it is properly coded to deal with this possibility. Also, properties files cannot be used on diskless computing platforms. Because of these problems, the Preferences API has been introduced as a replacement for the Properties API. The Preferences API lets you store preferences in a hierarchical manner so that you can avoid name collisions. Because this API is backend-neutral, it does not matter where the preferences are stored (a file, a database, or [on Windows platforms] the registry); you do not have to hardcode file names and locations. Also, there are no text-based files that can be modified, and Preferences can be used on diskless platforms. This API uses trees of nodes to manage preferences. These nodes are the analogue of a hierarchical filesystem’s directories. Also, preference name/value pairs stored under these nodes are the analogues of a directory’s files. You navigate these trees in a similar manner to navigating a filesystem: specify an absolute path starting from the root node (/) to the node of interest; for example, /window/location and /window/size. There are two kinds of trees: system and user. All users share the system preference tree, whereas the user preference tree is specific to a single user, which is generally the person who logged into the underlying operating system. (The precise description of “user” and “system” varies from implementation to implementation of the Preferences API.) Although the Preferences API’s java.util.prefs package contains three interfaces (NodeChangeListener, PreferencesChangeListener, and PreferencesFactory), four regular classes (AbstractPreferences, NodeChangeEvent, PreferenceChangeEvent, and Preferences), and two exception classes (BackingStoreException and InvalidPreferencesFormatException), you mostly work with the Preferences class. The Preferences class describes a node in a tree of nodes. To obtain a Preferences node, you must call one of the following utility methods: CHAPTER 9: Discovering Additional Utility APIs 430  static Preferences systemNodeForPackage(Class c): Return the node whose path corresponds to the package containing the class represented by c from the system preference tree.  static Preferences systemRoot(): Return the root preference node of the system preference tree.  static Preferences userNodeForPackage(Class c): Return the node whose path corresponds to the package containing the class represented by c from the current user’s preference tree.  static Preferences userRoot(): Return the root preference node of the current user’s preference tree. Listing 9–26 demonstrates systemNodeForPackage(), along with Preferences’ abstract void put(String key, String value) and abstract String get(String key, String def) methods for storing String-based preferences to and retrieving String-based preferences from the system preference tree. A default value must be passed to get() in case no value is associated with the key (which should not happen in this example). Listing 9–26. Storing a single preference to and retrieving a single preference from the system preference tree package ca.mb.javajeff.examples; import java.util.prefs.Preferences; public class PrefsDemo { public static void main(String[] args) { Preferences prefs = Preferences.systemNodeForPackage(PrefsDemo.class); prefs.put("version", "1.0"); System.out.println(prefs.get("version", "unknown")); } } When you run this application (java ca.mb.javajeff.examples.PrefsDemo), it generates the following output: 1.0 More interestingly, Figure 9–8 reveals how this preference is stored in the Windows XP registry. Figure 9–8. Listing 9–26’s version key in a Windows XP registry context CHAPTER 9: Discovering Additional Utility APIs 431 My Computer\HKEY_LOCAL_MACHINE\SOFTWARE\JavaSoft\Prefs is the path to the Windows XP registry area for storing system preferences. Under Prefs, you will find a node for each package stored in this area. For example, ca identifies Listing 9–26’s ca package. Continuing, a hierarchy of nodes is stored under ca. Within the bottommost node (examples), you find an entry consisting of version (the key) and 1.0 (the value). Listing 9–27 provides a second example that works with the user preference tree and presents a more complex key. Listing 9–27. Storing a single preference to and retrieving a single preference from the current user’s preference tree import java.util.prefs.Preferences; public class PrefsDemo { public static void main(String[] args) { Preferences prefs = Preferences.userNodeForPackage(PrefsDemo.class); prefs.put("SearchEngineURL", "http://www.google.com"); System.out.println(prefs.get("SearchEngineURL", "http://www.bing.com")); } } When you run this application (java PrefsDemo), it generates the following output: http://www.google.com More interestingly, Figure 9–9 reveals how this preference is stored in the Windows XP registry. Figure 9–9. Listing 9–27’s SearchEngineURL key in a Windows XP registry context The Windows XP registry encodes the SearchEngineURL key into /Search/Engine/U/R/L because Preferences regards keys and node names as case sensitive but the Windows XP registry does not. NOTE: Check out Ray Djajadinataz’s “Sir, What Is Your Preference?” article (http://www.javaworld.com/javaworld/jw-08-2001/jw-0831-preferences.html) to learn more about the Preferences API. CHAPTER 9: Discovering Additional Utility APIs 432 Random Number Generation Chapter 6 formally introduced you to the java.lang.Math class’s random() method. If you were to investigate this method’s source code, you would discover Listing 9–28’s implementation (for Java SE 6 Update 16). Listing 9–28. Implementing Math’s random() method public static double random() { if (randomNumberGenerator == null) initRNG(); return randomNumberGenerator.nextDouble(); } private static Random randomNumberGenerator; private static synchronized void initRNG() { if (randomNumberGenerator == null) randomNumberGenerator = new Random(); } Listing 9–28 shows you that Math’s random() method is implemented in terms of a class named Random, which is located in the java.util package. Random instances generate sequences of random numbers and are known as random number generators. NOTE: These numbers are not truly random because they are generated from a mathematical algorithm. As a result, they are often referred to as pseudorandom numbers. However, it is often convenient to drop the “pseudo” prefix and refer to them as random numbers. Also, delaying object creation (new Random(), for example) until the first time the object is needed is known as lazy initialization. Random generates its sequence of random numbers by starting with a special 48-bit value that is known as a seed. This value is subsequently modified by a mathematical algorithm, which is known as a linear congruential generator. NOTE: Check out Wikipedia’s “Linear congruential generator” entry (http://en.wikipedia.org/wiki/Linear_congruential_generator) to learn about this algorithm for generating random numbers. Random declares a pair of constructors:  Random() creates a new random number generator. This constructor sets the seed of the random number generator to a value that is very likely to be distinct from any other call to this constructor.  Random(long seed) creates a new random number generator using its seed argument. This argument is the initial value of the internal state of CHAPTER 9: Discovering Additional Utility APIs 433 the random number generator, which is maintained by the protected int next(int bits) method. Because Random() does not take a seed argument, the resulting random number generator always generates a different sequence of random numbers. This explains why Math.random() generates a different sequence each time an application starts running. TIP: Random(long seed) gives you the opportunity to reuse the same seed value, allowing the same sequence of random numbers to be generated. You will find this capability useful when debugging a faulty application that involves random numbers. Random(long seed) calls the void setSeed(long seed) method to set the seed to the specified value. If you call setSeed() after instantiating Random, the random number generator is reset to the state that it was in immediately after calling Random(long seed). Listing 9–28 demonstrates Random’s double nextDouble() method, which returns the next pseudorandom, uniformly distributed double precision floating-point value between 0.0 and 1.0 in this random number generator’s sequence. Random also declares the following methods for returning other kinds of values:  boolean nextBoolean() returns the next pseudorandom, uniformly distributed Boolean value in this random number generator’s sequence. Values true and false are generated with (approximately) equal probability.  void nextBytes(byte[] bytes) generates pseudorandom byte integer values and stores them in the bytes array. The number of generated bytes is equal to the length of the bytes array.  float nextFloat() returns the next pseudorandom, uniformly distributed floating-point value between 0.0 and 1.0 in this random number generator’s sequence.  double nextGaussian() returns the next pseudorandom, Gaussian (“normally”) distributed double precision floating-point value with mean 0.0 and standard deviation 1.0 in this random number generator’s sequence.  int nextInt() returns the next pseudorandom, uniformly distributed integer value in this random number generator’s sequence. All 232 possible integer values are generated with (approximately) equal probability.  int nextInt(int n) returns a pseudorandom, uniformly distributed integer value between 0 (inclusive) and the specified value (exclusive), drawn from this random number generator’s sequence. All n possible integer values are generated with (approximately) equal probability. CHAPTER 9: Discovering Additional Utility APIs 434  long nextLong() returns the next pseudorandom, uniformly distributed long integer value in this random number generator’s sequence. Because Random uses a seed with only 48 bits, this method will not return all possible 64-bit long integer values. The java.util.Collections class declares a pair of shuffle() methods for shuffling the contents of a list. In contrast, the java.util.Arrays class does not declare a shuffle() method for shuffling the contents of an array. Listing 9–29 addresses this omission. Listing 9–29. Shuffling an array of integers import java.util.Random; public class Shuffler { public static void main(String[] args) { Random r = new Random(); int[] array = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }; for (int i = 0; i < array.length; i++) { int n = r.nextInt(array.length); // swap array[i] with array[n] int temp = array[i]; array[i] = array[n]; array[n] = temp; } for (int i = 0; i < array.length; i++) System.out.print(array[i] + " "); System.out.println(); } } Listing 9–29 presents a simple recipe for shuffling an array of integers—this recipe could be generalized. For each array entry from the start of the array to the end of the array, this entry is swapped with another entry whose index is chosen by int nextInt(int n). When you run this application, you will observe a shuffled sequence of integers that is similar to the following sequence that I observed: 5 8 3 4 0 2 7 6 9 1 Regular Expressions API Text-processing applications often need to match text against patterns. For example, an application might need to locate all occurrences of a word in a text file so that it can replace those occurrences with another word. Java provides regular expressions to help text-processing applications perform pattern matching. A regular expression (also known as a regex or regexp) is a string-based pattern that represents the set of strings that match this pattern. The pattern consists of literal characters and metacharacters, which are characters with special meanings instead of literal meanings. This book was purchased by harshamv@variable3.comCHAPTER 9: Discovering Additional Utility APIs 435 The Regular Expressions API provides the java.util.regex.Pattern class to represent patterns via compiled regexes. Regexes are compiled for performance reasons; pattern matching via compiled regexes is much faster than if the regexes were not compiled. Table 9–4 describes Pattern’s methods. Table 9–4. Pattern Methods Method Description static Pattern compile (String regex) Compile regex and return its Pattern object. This method throws java.util.regex.PatternSyntaxException when regex’s syntax is invalid. static Pattern compile (String regex,int flags) Compile regex according to the given flags (a bitset consisting of some combination of Pattern’s CANON_EQ, CASE_INSENSITIVE, COMMENTS, DOTALL, LITERAL, MULTILINE, UNICODE_CASE, and UNIX_LINES constants) and return its Pattern object. This method throws PatternSyntaxException when regex’s syntax is invalid, and IllegalArgumentException when bit values other than those corresponding to the defined match flags are set in flags. int flags() Return this Pattern object’s match flags. This method returns 0 for Pattern instances created via compile(String), and the bitset of flags for Pattern instances created via compile(String, int). Matcher matcher (CharSequence input) Return a Matcher that will match input against this Pattern’s compiled regex. static boolean matches (String regex, CharSequence input) Compile regex and attempt to match input against the compiled regex. Return true if there is a match; otherwise, return false. This convenience method is equivalent to Pattern.compile(regex).matcher(input).matches(), and throws PatternSyntaxException when regex’s syntax is invalid. String pattern() Return this Pattern’s uncompiled regex. static String quote (String s) Quote s using "\Q" and "\E" so that all other metacharacters lose their special meaning. If the returned String is later compiled into a Pattern instance, it can only be matched literally. String[] split (CharSequence input) Split input around matches of this Pattern’s compiled regex and return an array containing the matches. String[] split (CharSequence input, int limit) Split input around matches of this Pattern’s compiled regex; limit controls the number of times the compiled regex is applied and thus affects the length of the resulting array. String toString() Return this Pattern’s uncompiled regex. CHAPTER 9: Discovering Additional Utility APIs 436 Table 9–4 reveals the java.lang.CharSequence interface, which describes a readable sequence of char values. Instances of any class that implements this interface (such as String, StringBuffer, and StringBuilder) can be passed to Pattern methods that take CharSequence arguments (such as split(CharSequence)). NOTE: CharSequence declares methods char charAt(int index) (return the character at location index within this sequence), int length() (return the length of this sequence), CharSequence subSequence(int start, int end) (return a subsequence of this sequence ranging from location start, inclusive, to location end, exclusive), and String toString() (return a string containing this sequence’s characters in the same order and having the same length as this sequence). Table 9–4 also reveals that each of Pattern’s compile() methods and its matches() method (which calls the compile(String) method) throws PatternSyntaxException when a syntax error is encountered while compiling the pattern argument. Table 9–5 describes PatternSyntaxException’s methods. Table 9–5. PatternSyntaxException Methods Method Description String getDescription() Return a description of the syntax error. int getIndex() Return the approximate index of where the syntax error occurred in the pattern, or -1 if the index is not known. String getMessage() Return a multiline string containing the description of the syntax error and its index, the erroneous pattern, and a visual indication of the error index within the pattern. String getPattern() Return the erroneous pattern. Finally, Table 9–4’s Matcher matcher(CharSequence input) method reveals that the Regular Expressions API also provides the java.util.regex.Matcher class, whose matchers attempt to match compiled regexes against input text. Matcher declares the following methods to perform matching operations:  boolean matches() attempts to match the entire region against the pattern. If the match succeeds, more information can be obtained by calling Matcher’s start(), end(), and group() methods. For example, int start() returns the start index of the previous match, int end() returns the offset of the first character following the previous match, and String group() returns the input subsequence matched by the previous match. Each method throws java.lang.IllegalStateException when a match has not yet been attempted or the previous match attempt failed. CHAPTER 9: Discovering Additional Utility APIs 437  boolean lookingAt() attempts to match the input sequence, starting at the beginning of the region, against the pattern. As with matches(), this method always starts at the beginning of the region. Unlike matches(), lookingAt() does not require that the entire region be matched. If the match succeeds, more information can be obtained by calling Matcher’s start(), end(), and group() methods.  boolean find() attempts to find the next subsequence of the input sequence that matches the pattern. It starts at the beginning of this matcher’s region, or, if a previous call to this method was successful and the matcher has not since been reset (by calling Matcher’s Matcher reset() or Matcher reset(CharSequence input) method), at the first character not matched by the previous match. If the match succeeds, more information can be obtained by calling Matcher’s start(), end(), and group() methods. NOTE: A matcher finds matches in a subset of its input called the region. By default, the region contains all of the matcher’s input. The region can be modified by calling Matcher’s Matcher region(int start, int end) method (set the limits of this matcher’s region), and queried by calling Matcher’s int regionStart() and int regionEnd() methods. I have created a simple application that demonstrates Pattern, PatternSyntaxException, and Matcher. Listing 9–30 presents this application’s source code. Listing 9–30. Playing with regular expressions import java.util.regex.Matcher; import java.util.regex.Pattern; import java.util.regex.PatternSyntaxException; public class RegExDemo { public static void main(String[] args) { if (args.length != 2) { System.err.println("usage: java RegExDemo regex input"); return; } try { System.out.println("regex = " + args[0]); System.out.println("input = " + args[1]); Pattern p = Pattern.compile(args[0]); Matcher m = p.matcher(args[1]); while (m.find()) System.out.println("Located [" + m.group() + "] starting at " + m.start() + " and ending at " + (m.end()-1)); } catch (PatternSyntaxException pse) { CHAPTER 9: Discovering Additional Utility APIs 438 System.err.println("Bad regex: " + pse.getMessage()); System.err.println("Description: " + pse.getDescription()); System.err.println("Index: " + pse.getIndex()); System.err.println("Incorrect pattern: " + pse.getPattern()); } } } After compiling this source code, execute java RegExDemo ox ox. You will discover the following output: regex = ox input = ox Located [ox] starting at 0 and ending at 1 find() searches for a match by comparing regex characters with the input characters in left-to-right order, and returns true because o equals o and x equals x. Continuing, execute java RegExDemo box ox. This time, you will discover the following output: regex = box input = ox find() begins by comparing regex character b with input character o. Because these characters are not equal, and because there are not enough characters in the input to continue the search, find() does not output a “Located” message to indicate a match. However, if you execute java RegExDemo ox box, you will discover a match: regex = ox input = box Located [ox] starting at 1 and ending at 2 The ox regex consists of literal characters. More sophisticated regexes combine literal characters with metacharacters (such as the period [.]) and other regex constructs. TIP: To specify a metacharacter as a literal character, precede the metacharacter with a backslash character (as in \.), or place the metacharacter between \Q and \E (as in \Q.\E). In either case, make sure to double the backslash character when the escaped metacharacter appears in a string literal; for example, "\\." or "\\Q.\\E". The period metacharacter matches all characters except for the line terminator (a one- or two-character sequence designating the end of the line). For example, each of java RegExDemo .ox box and java RegExDemo .ox fox report a match because the period matches the b in box and the f in fox. CHAPTER 9: Discovering Additional Utility APIs 439 NOTE: Pattern recognizes the following line terminators: carriage return (\r), newline (line feed) (\n), carriage return immediately followed by newline (\r\n), next line (\u0085), line separator (\u2028), and paragraph separator (\u2029). The period metacharacter can be made to also match these line terminators by specifying the Pattern.DOTALL flag when calling Pattern.compile(String, int). A character class is a set of characters appearing between [ and ]. There are six kinds of character classes:  A simple character class consists of literal characters placed side by side, and matches only these characters. For example, [abc] consists of characters a, b, and c. Also, java RegExDemo t[aiou]ck tack reports a match because a is a member of [aiou]. It also reports a match when the input is tick, tock, or tuck because i, o, and u are members.  A negation character class consists of a circumflex metacharacter (^), followed by literal characters placed side by side, and matches all characters except for the characters in the class. For example, [^abc] consists of all characters except for a, b, and c. Also, java RegExDemo "[^b]ox" box does not report a match because b is not a member of [^b], whereas java RegExDemo "[^b]ox" fox reports a match because f is a member. (The double quotes surrounding [^b]ox are necessary on my Windows XP platform because ^ is treated specially at the command line.)  A range character class consists of successive literal characters expressed as a starting literal character, followed by the hyphen metacharacter (-), followed by an ending literal character, and matches all characters in this range. For example, [a-z] consists of all characters from a through z. Also, java RegExDemo [h-l]ouse house reports a match because h is a member of the class, whereas java RegExDemo [h-l]ouse mouse does not report a match because m lies outside of the range and is therefore not part of the class. You can combine multiple ranges within the same range character class by placing them side by side; for example, [A-Za-z] consists of all uppercase and lowercase Latin letters.  A union character class consists of multiple nested character classes, and matches all characters that belong to the resulting union. For example, [abc[u-z]] consists of characters a, b, c, u, v, w, x, y, and z. Also, java RegExDemo [[0-9][A-F][a-f]] e reports a match because e is a hexadecimal character. (I could have alternatively expressed this character class as [0-9A-Fa-f] by combining multiple ranges.) CHAPTER 9: Discovering Additional Utility APIs 440  An intersection character class consists of multiple &&-separated nested character classes, and matches all characters that are common to these nested character classes. For example, [a-c&&[c-f]] consists of character c, which is the only character common to [a-c] and [c-f]. Also, java RegExDemo "[aeiouy&&[y]]" y reports a match because y is common to classes [aeiouy] and [y].  A subtraction character class consists of multiple &&-separated nested character classes, where at least one nested character class is a negation character class, and matches all characters except for those indicated by the negation character class/classes. For example, [a- z&&[^x-z]] consists of characters a through w. (The square brackets surrounding ^x-z are necessary; otherwise, ^ is ignored and the resulting class consists of only x, y, and z.) Also, java RegExDemo "[a- z&&[^aeiou]]" g reports a match because g is a consonant and only consonants belong to this class. (I am ignoring y, which is sometimes regarded as a consonant and sometimes regarded as a vowel.) A predefined character class is a regex construct for a commonly specified character class. Table 9–6 identifies Pattern’s predefined character classes. Table 9–6. Predefined Character Classes Predefined Character Class